Muzan's blood may also have an effect on a demon's personality, making them as monstrous as he is. His blood grants them immense power, at the cost of horrifying mutations and mass deportation from the lands of rationality. Most demons seen were at least decent people as humans before they became demons.
It's suggested that to become a Hashira, the slayer must be either extremely skilled or have slain one of the Twelve Kizuki. Rengoku states that his family has a history of producing Flame Hashira. The side notes also state that every generation of Hashira has a Flame and Water Hashira. It's very likely that Rengoku's family keep having the huge misfortune of encountering one of the Twelve Kizuki (even if they might be Lower Ranked) - and those demons are the ones with the highest concentrations of Muzan's blood. It's a wonder his family line hasn't been killed off.
This is lessened somewhat since it's revealed that the first method to become a Hashira is accumulating a demon kill count of at least 50.
Black swords are mentioned to be a bad omen... more accurately for the demons. It turns out black swords are for swordsmen who can use the Breath of the Sun and Kokushibo mentioned that he and Muzan hunted and killed every Sun Breath user they could find, so it's only natural that black swords are considered a bad omen. By killing them all, it's small wonder that the Demon Slayer Corps and their associates believe that no swordsmen with a black sword will ever be a Hashira.
It's pointed out by Kokushibou that those with the Demon Slayer Mark do not live past 25. Even though they achieved their happy ending, the surviving Demon Slayers who received the mark will have a limited time to live, meaning Tanjirou will eventually leave Nezuko, Inosuke and Zenitsu, while also leaving Kanao a widow. However, since Yoriichi proves that exceptions to the rule can exist, there is still a slim chance that Tanjiro can somehow accomplish the same feat, although only time will tell.
That being said, there is still hope. It’s revealed in the end that Muzan’s death lifted the curse on the Ubuyashiki family causing Kiriya to become the oldest man in Japan when otherwise he would have died on his 20s. Assuming that the Mark’s lethality is also the result of one of Muzan’s insults against the world, then the marked slayers may have survived.
Similarly to the above, assuming that the early death caused by the Demon Slayer Mark is still in effect, both Giyu and Sanemi, both of whom are 21 years old, only lived four years after Muzan's defeat at most. That's bad enough, but it gets worse when one remembers that both of them have descendants post-time skip. Descendants that look nearly identical to them, so they're most likely not adopted. Neither are shown to have a girlfriend within the series, so presumably they both met a woman they liked, fell in love with them, got married, and conceived a child... within the last four years of their lives. In which case they both left behind a widow who they only got to know for four years maximum, in addition to leaving them as single mothers, and left behind at least one child who at the oldest was only three years old.
Considering Tamayo's beauty, it is not unlikely that Muzan was keeping her as an demonic concubine. In fact, the two were seen traveling together during their encounter with Yoriichi (with Tamayo sporting a particularly submissive and worn expression no less), which is something Muzan is never seen doing with any other demon, and Tamayo doesn't appear to be much of a fighter or hunter anyway, implying that Muzan kept her around for other reasons...
Just how many people have the Demons collectivelly killed? Just for scale Muzan has lived for roughly 1000 years. If Muzan killed a single person per month (which would be uncannily nice of him), that means he has 12.000 bodies in his count. And that's not getting into the thousands of lesser demons he sired...
The setting states that demons don't normally work together and are very vindictive of other demons entering their turf. Even the Upper Moons don't seem to get along at all and mostly just listen to Muzan. However, if Yahaba and Susamaru and especially Daki and Gyutaro are any indication, two demons in any kind of snyc is a lot to go up against. If Muzan hadn't deliberately discouraged teamwork with how he makes demons, the Demon Slayer Corps would've almost certainly been slaughtered long before Tanjiro took up the sword. In fact, there are countless oppurtunites in which demons working could've have spelled more deaths at best and The Bad Guy Wins at worst:
Kyogai captured a rare blood and was planning on eating him, only for two demons to storm in on his turf to fight him over it. Even worse for him, one demon tore off a drum from his body, the one for teleportation around the house, which the kid immediately uses to escape and hide from all of them. Had the concept of sharing even crossed Kyogai's head, he still would've have gotten over 10 humans worth of blood from splitting him in 3, and kept his most useful drum, potentially giving him enough strength to kill Tanjiro, which likely would've gotten him back in Muzan's good graces right quick.
The Spider Family in particular could've slaughtered the good guys with ease. It's explicitly stated they were all weak demons that wanted companions and thus they took Rui's offer, only to get it in the worst way possible. Tanjiro and Inosuke still straight up lost to their opponents, and Zenitsu won by a hair. Even a remotely functional "family" would've been far too much for them all. Hell, something as simple as the Elder Brother Demon poisoning Tanjiro and Inosuke right when the Father Demon dropped down on them or the Daughter Demon staying to spray them with their webs could've been enough to kill without Rui even getting involved. To say nothing of how easily all of them would've been killed by Father and Rui working together.
The fact that the Father Spider Demon is a Dumb Muscle whose one goal in his demon life is to single-trackedly defend his "family" becomes more disturbing once you remember what Rui was stated to do to those among his "family" who refuse or fail to play along with his fantasies: besides being carved up by his webs or being left to burn in the sunlight, they also have their intellect be taken away. In other words, Father Spider Demon has essentially been lobotomized by Rui at some point in the past, reducing him to a mindless brute obsessed with "protecting his family". Even if he may have possibly been an Asshole Victim, that still says a lot about what kind of fate would befall upon anyone who dares to piss off Rui in any way or form.
It is heavily implied that Uzui married Hinatsuru, Makio, and Suma when they were 13, 12, and 11 respectively and he himself was only 15. This is very young even for the Taisho Era, when the legal marrying age was 16 for girls and 19 for boys. Obviously it worked out in their case since Uzui values consent and legitimately cares for his wives, but when put in the context of the shinobi and how they view women as expendable baby-factories, it's extremely disturbing. Most marriages are arranged too, so these girls don't get much choice in the matter. And let's not forget that Uzui was supposed to marry Suma's younger sister.
Imagine you're a john in the Entertainment District. If you have the rotten luck of picking Daki to spend the night, you not only have a more-than-insignificant chance of getting devoured (maybe alive), Gyutarowas there the whole time, watching, and ready to kill you if you managed to escape Daki by some true miracle.
The end of the penultimate chapter has Tanjiro hoping that they can live in peace forever. However, Tanjiro lives in the Taisho era around 1920s, so Japan wouldn't have peace for a long time.
As a human, Gyokko, then named Managi, thought the drowned bodies of his parents where "beautiful." This is already disturbing enough, but given that it's very possible that the corpses were bloated due to dying in the water it makes it 10 times more horrific.
