They have too much power in this situation to back down. It doesn't really matter what Democrats in other states think of Manchin or Sinema. He faces an electorate that is 2/3 Trumpists, hers is 50-50. So long as you have a donor base in post Citizens United DC, you have power.
Then on the flip side Biden has burned most of his goodwill with Independents while the Dems whine about how unfair the media is (something else they have in common with their GOP counterparts) to their guy and spin facts as brutally as any Republican.
It sucks that America can't have nice things. But pretending for months that Manchin and Sinema have any incentive to cave until they get all the precious things they want was frankly criminal given the stakes the Democrats themselves have laid for Biden's Administration.
Despite the real threat of Trump, they haven't changed. Mark my words, when they lose the House and then have GOP state legislatures install Trump for a second term, the leading Democrats will be rallying a second "Resistance" by calling for MOAR DONOR DOLLARS and us putting ourselves in front of the jackbooted thugs while they pass Trump's budgets.