1) Democrats in general do not agree that there is no leadership. It is clear that generally speaking Democrats are all of the adults in the room right now, and it won't matter much which Democrat is the nominee. Biden is polling the best as far as beating Trump, but Warren or Bernie would beat him also. No other Democrat will win the nomination.
2) Romney is 72. He was just elected as a senator. He has already lost one bid for the Presidency. I'm not sure he will want to be a two-time loser. If Warren is the Democrat nominee, she is younger. As a woman, statistically she will live longer. Romney would lose.
3) There is too much GOP damage. Trump has damaged the GOP for at least one term for a Democrat in the White House. A GOP candidate might have had a chance if the GOP stood up immediately upon finding he made a deal with Ukraine to have them find dirt on the Bidens. They just look SOOOO bad now. Current Trump approval poll from RCP is 42.7. It was at 45.3 when this Ukraine stuff started. The PARTY takes the hit for that. Yes, there will be a handful of GOP voters who wouldn't vote for Trump but would for Romney, but there are a lot more Trump voters who will just stay home. Romey isn't anti-establishment. Romney isn't entertaining. Many of those Trump voters vote for Trump because of those things.
There will be a Democrat in the White House beginning Jan. 2021. No Republican can do it this time. Trump ruined that.