Last year at this time, I was of the view that, while the case for conviction might be a winning one on the merits, there was no actual likelihood of conviction, and that, therefore, there was little point in proceeding with impeachment. That being the case, it might be preferable, I believed then, to leave the decision to the American voters, who did indeed do in November what the Senate had failed to do in February. (At what cost, however, given all that transpired between February and November!)
Now, however, the issue is presidential misbehavior after the election, for which the the only remaining political sanction would seem to be impeachment, conviction, and disqualification. The 1876 impeachment and trial of Secretary of War, William Belknap, provides a precedent that leaving office does not confer immunity against impeachment and its possible consequences. But Belknap was acquitted, as Trump seems now likely also to be, which he and his supporters will presumably again interpret as exoneration.
Of course, the trial could have been begun while Trump was still in office, if the then-Leader McConnell had not delayed the proceedings. Yesterday's vote has now confirmed that there is not much likelihood of the Republican party changing its course, even after an incited mob's assault on the Capitol.
That said, what now is the point of proceeding to Trump's second trial, besides guaranteeing permanent historical opprobrium for Trump? Presumably, impeachment itself is something of a sanction. The mere prospect of leaving office having been impeached may indeed deter some presidents from such last-minute bad behavior, a worthwhile deterrent to be sure.
In the long-term, however, the real deterrent would be to elect better presidents, who accept the norms of law and constitutional democratic governance - and, while we're at it, Senators who do too!
Meanwhile, absent an unusual bipartisan consensus, the likes of which should not be expected in our currently polarized politics, it would seem that the impeachment provisions in the Constitution (as far as presidents are concerned) have become moot again, as they were for over 100 years after the unsuccessful precedent of 1868.