I definitely agree that they don't have a moral high ground here. And there is no doubt Congress has become more about partisanship than the good of the people. However, there are some important things to consider here:
Pelosi did NOT WANT to impeach Trump. From the beginning she fought off any attempt by other Democrats to begin such a process. I do believe that Trump's actions crossed the line from doing what's good for the people (at least those that elected him) to doing what's good for his political re-election, and that should be a VERY clear line that is not permitted. Dershowitz' claim that "he believed he was doing the good for the country by helping his re-election" is just bizarre giving the types of actions that statement justifies. NO president should put themselves as being more important than the sanctity of the democratic institution. Terms are limited; the system must live on intact. I believe Pelosi made the right call here.
Remember also that she made this call even given two important facts: she KNEW that the impeachment would not succeed, and the failed attempt at impeaching Clinton only served to boost his support. This was a very risky endeavor. To claim that it's an attempt to "undo the will of the people" totally ignores the facts: it was never going to succeed, and trying this in an election year actually increases his chances for re-election.
In terms of precedent, I think the bigger harm is on the side of McConnell: openly declaring that he would work in lockstep with the White House, and not allowing any witnesses have removed the last vestiges of an impartial process for impeachment.