They want to drag it out to have maximum effect on the 2020 election. However, the effect will be in Trump's favor and they are beginning to realize what they have stepped into. ----What they are really concerned with is not just the 2020 election but the fact that either Pence or another Trump will then follow and that they will not get power back for a long long time.
Well, the other thing there is- just playing along with their little impeachment fantasy- if Trump is removed by impeachment, it is not a given that Pence takes his place on the ticket. Neither Pence nor Trump have been nominated just because there is no primary. They are not the nominee until the convention votes them in and even if it is past primary time, they can still run a brokered convention right there on the floor and settle it.
What I am getting at is if Trump were removed, all the usual suspects would come out to run before the convention: Rubio, Cruz, Kasich, etc. Some of them would clearly be competitive against Warren- most people are.