Introspection is the hardest thing to do for parties, tbh... Dems were lucky to avoid doing it when Magic Negro showed up to pull a Trump before Donny did (and so, as you say, probably weren't aware of their precariousness).
But the GOP isn't off the hook either, IMO. This election was like Mondale vs Mondale. The two tiest candidates I can remember. The hope is that such terrible process (including the rigging job on the Dems side) forces more than just 30% of eligible voters to get involved in the primary process.
There's also one good thing that comes from losing, and you've seen this in the GOP too: there should now be a much bigger gamut of candidates on the Democratic side to pick from. Heck, arguably, the more you lose the more options you get.