Strange isn't virtually omnipotent anymore, but all of his old opponents still are, and there is no hero with both the scale of power, skill, and stability to reliably stand against them anymore. Brother Voodoo, The Sentry, and Molecule Man are dead, Legion is insane, Scarlet Witch is recovering but still mentally fragile, Wiccan is still coming into his power, Hulk is a one-trick pony, generally nowhere near his infinite upper levels of power, and insane, Franklin Richards is still a child, The Mighty Thor doesn't possess the Odinforce anymore, The Incredible Hercules only briefly possessed that scale of power, Nico Minoru is too inexperienced, and powerful as the Silver Surfer may be, he couldn't even make Dormammu notice his attacks. Finally resolved with the end of Bendis' New Avengers, which saw him restored to his title as Sorcerer Supreme. Though the idea of a Sorcerer Supreme — one solitary man or woman protecting the Earth against unspeakable threats — is scary in and of itself.
Fortunately, he has the gods on his side. All of them. Though that's only usually enough. And he has a long line of people ready to take his place the moment he falls.
