Back then, like today, Dems (and everyone else) had no idea if pursuing impeachment would change those numbers, or backfire.
The idea that they had a cooperative GOP for impeachment is a myth.
Folks also could have argued Nixon had advantages his GOP predecessors didn't have, in terms of burgeoning support in the Democratic south, new conservative influencers on the scene, etc.
Dems can either convince themselves that the risks aren't hypothetical, but real as the sun rising, or they can say they do not know, and the only certain thing is that they're running out of options, as Trump defies subpoenas.
They DID know they didn't have polls, or Senate GOP votes, on their side.
But, they did what was right, and polls slowly joined them.