It is worth pointing out that Twitter had concrete near-term reasoning around incitement of further attacks on the capitol that they included in their reasoning - incitement is generally not protected speech (even though of course Twitter is a private platform and not restricted by the First Amendment). It is a legitimate concern that Trump would tacitly or explicitly encourage further mob attacks on the Capitol:

"Plans for future armed protests have already begun proliferating on and off-Twitter, including a proposed secondary attack on the US Capitol and state capitol buildings on January 17, 2021.

As such, our determination is that the two Tweets above are likely to inspire others to replicate the violent acts that took place on January 6, 2021, and that there are multiple indicators that they are being received and understood as encouragement to do so."

https://blog.twitter.com/en_us/topics/company/2020/suspensio...

I also understand, even if it's not consistent, that Twitter would maintain a policy-violating account if it is that of a major world leader on public interest grounds alone. Once he is no longer the head of government, such reasons no longer apply. Practically speaking, this reason for maintaining the account the last two weeks of his presidency was, I suspect, outweighed by the immediacy of the worry on further insurrection attempts. I fall into the small camp of people (it seems?) that thinks Twitter was right not to have suspended his account earlier, and is right to now.