I'd buy an argument that the national party didn't want to make this into a race since they knew voter enthusiasm was minimal, although I'd point out that a single tweet from Biden (fuck, where was Beto?) would have sent Sanchez to the runoff. No need to endorse anyone. Just remind people to vote. An extra 1500 blue votes would have done it.
I'd rather not focus on the national party anyway, as I'm most concerned about Dem voter enthusiasm, or lack thereof. The fact is that the national party wasn't needed to win elections for Democrats during the Trump era. Add to the fact that it's more difficult for the Democratic party to manufacture enthusiasm among its supporters than it is for Republicans, and I'm not convinced things are just going to work themselves out in 2022.
You can't just look at the win column. Democrats outperformed across the board in 2017, even if they came up short in flipping some deeply red districts left vacant by Trump appointees. Tom Price won GA-6 by 23 points in 2016. Ossoff came short by only 4 points a year later. Kansas, Montana, the VA elections, and of course Alabama Senate. Not all wins, but the trend was obvious.
2009 was a similar story for the Republicans. They lost close races trying to fill vacancies of Obama appointees, but those races shouldn't have been close on paper. It didn't take long for them to start winning.