As best I can tell, while Trump is morally responsible for the recent riot he is not legally responsible, since everything he did that contributed to it was something he had a legal right to do. But requirements for impeachment, other than a majority vote in the House to impeach and two-thirds in the Senate to convict, are unclear, so that is not, in my view, the fundamental issue.
Our legal system has so far been pretty stable. One reason is an implicit rule: When power shifts, the winners don't punish the losers. Impeaching Trump after he has left office, as a punishment not a way of removing him from power, violates that rule. That would be a dangerous precedent, one step further towards making political conflict something closer to a civil war.
People will, of course, argue that this is a special case, that Trump is uniquely guilty. But once the precedent is established other people, in a polity already sharply divided, will find other special cases.
For the same reason I am bothered by people who gloat over the prospect that Trump, once out of office, will be prosecuted for things he did in business before he was president. Obviously having been president doesn't give legal immunity — anyone who wants to sue him will be, and should be, free to do so. But criminal prosecution is at the discretion of the prosecutor; Obama protected illegal immigrants who he didn't want arrested by instructing law enforcement not to arrest them. If Trump gets prosecuted by officials who are his political enemies for business dealings he did not get prosecuted for when they happened, it will be pretty clear that it isn't the dealings he is being prosecuted for.
That again would be an unfortunate precedent.