The problem is electing someone as President who would use Twitter the way Trump does. We should never have done that in the first place and, thankfully, that's an error We The People can correct.
Addendum:
I do, though, think that Twitter's response is inadequate. They have a community. Their community is being turned toxic by this man and his ilk. His use of Twitter as President isn't the problem, but Twitter needs to address the effect of people like him on their platform and the world in general.
And that way more complicated than just banning a warmonger President. Sadly, I'm not sure Twitter has shown themselves to be up to the challenge.