They don't have any skin in the game though, not in comparison to the moderates (in moderate districts*) that have come forward in support of impeachment. Again, the relevance of Maxine Waters to this discussion is wanting. Even if politically she's a moderate the calculus changes when standing atop her +29 Democratic leaning California district. Her prematurely calling for impeachment in that environment doesn't speak to a bravery or leadership so much as a freedom that other members in her caucus haven't had up until now. It's not a decision that she is making with the threat of potentially losing her seat for it in the 2020 cycle.
The Mueller Probe didn't begin until May 17, 2017.
Donald Trump has been in violation of the emoluments clause since the day he took office.
Donald Trump fired James Comey, and then admitted on national television that he fired Comey because of the Russia Investigation in an interview with Lester Holt on May 11, 2017.
Donald Trump, weekly, behaves in a manner unfitting the Presidency (also a standard of impeachment), damn near weekly.
The facts reject your assertion that Donald Trump was only a candidate for impeachment post the Mueller Report. We're talking about a president who was in clear violation of the law well before the existence of the special investigation you're hailing as the standard.
I will also repeat that Maxine Waters has been the subject of constant harassment and death threats for, yes, her bravery and leadership.