Yes, the Democrats main worry for 2020 was defeating Trump. Biden was a safe candidate to do so. Trump was the great equalizer for intra-party disputes. All factions of the Democratic Party wanted him gone. Yes, I would say the Democratic Primaries and debates started off about ideas, then after South Carolina became all about defeating Trump.
One could say the Democrats won their big, main battle, but lost a lot of little battles down ballot. They pulled off the big victory but were considerably weaken by the down ballot battles they lost. Thus, causing the intra-party fighting as their numbers dwindled to a bare majority. One expects when a presidential candidate wins by 7 million plus votes, he brings along a lot of other democrats with him. Obama brought in 23 new democratic representatives and 8 senators in his 8 million vote win over McCain. Reagan brought in 34 new Republican representatives and 12 senators. Biden lost 13 democratic house members.
The thing is with intraparty in-fighting has always gone on. But usually there’s been a lot bigger margin in the house and in the senate to paper over that in-fighting which can’t be done with just a 9-seat majority in the house and a tie in the senate. Take Obama and the ACA, the Democrats had a 256-178 advantage, so when 34 house democrats voted against it, it still passed with votes to spare. I’m sure there was in-fighting back then also, but with a 78-seat majority, no one noticed. It didn’t matter but does with just a 9-seat majority 222-213.