Impeaching him at any point would be a valuable, if ultimately only symbolic, feather in their cap they could point to when addressing the more dogmatic sections of their base to drum up support for literally whatever they wanted.
This is it. I don’t like the guy at all but impeachment won’t do anything. Clinton was impeached and he finished his second term.
That said, fine. Then do something of actual value and strengthen your chances of winning. But they haven’t and I would bet they won’t. Dems often seem to want to coast across a finish line, not actually run the marathon.