Humility and contrition weren't exactly the route the Republicans used to get the pendulum to swing back their way again.
In UK politics Labour is a veritable orgy of soul-searching about what they did wrong after a slightly-less-narrow-than expected election defeat, which Brexit hasn't exactly improved. The SNP has a clear "voters were cheated by the lies of the London media" grievance narrative and hasn't shifted an inch on the desirability of an independent Scotland. One of these parties has seen the political pendulum swing back towards them.
Ultimately, if the Democrats are to win office again they'll do it from capturing votes from people angry enough about Trump to change their voting intention (and from having a different candidate) not from conceding he was right about a huge number of things. Sure, it might be a comfort blanket for their own mistakes, but the "Trump and his supporters deceived voters!" narrative is also at least as likely to succeed at winning over disaffected Trump/non-voters than trying to reinvent themselves by borrowing his policies or tone.