I thought about that a lot during the impeachment hearings, and the only explanation I can come up with that makes sense to me is that key Dems knew, from the very beginning, that it was doomed to fail. Even if they got an impeachment vote in the Democrat-dominated House, the Republican-dominated Senate could still vote to acquit.
They proceeded anyway, to humiliate Trump, create doubt and distrust for him among his base, and to stoke such intense anti-Trump sentiment among the left, they could guarantee themselves a win in 2020. When the Senate vote failed, they could just say, "Well, look at him! He's so slimy, and has obviously escaped justice before! And those Republicans, putting partisan politics above justice, letting him get away with it!" They figured they could get double-duty out of it, discrediting Senate Republicans (and possibly seeing some voted out of office as a result), as well as Trump.
So I think the impeachment went as planned--or at least as expected. The first mutterings of "Vote Blue, No Matter Who" kicked off during the impeachment hearings, and has been growing since. The media did their part in stoking anti-Trump sentiment. And that's how the 2020 election year kicked off.
Had the impeachment succeeded, we only would have had a year of Mike Pence as president--with Nancy Pelosi as his VP, and a Democrat as Speaker of the House. He would have been ruthlessly obstructed and made out to be an even bigger threat to democracy than Trump--look at the insane, rabid shit said about how doomed we all are with ACB on the Supreme Court because she's a religious conservative, and you get the idea.
Pence is very smart and disciplined, but he would have got nothing done, been condemned for his handling of the Covid-19 crisis and the BLM/Antifa riots no matter what he did, and would have been voted out in November and gone in January. He wouldn't have been able to campaign effectively while dealing with endless obstructivist bullshit, and frankly, he's not as charismatic as he needs to be if he's to be a viable presidential contender. He'd also spent most of the previous three years largely invisible, which wouldn't have helped.