If you think kneecapping outside candidates is inevitable, keep in mind that the far more competent, far more disciplined Republican party couldn't do it against Trump in 2016. Democrats were able to pull it off in 2020 because:

1. They had a candidate who was a popular, high-ranking official from a popular Democratic presidential administration.
2. That same candidate had enough of a real base of support to win a primary state despite looking dead in the water.
3. That same candidate was able to get not just the endorsement of a popular former president, but was able to get that former president to convince his more successful competitors to drop out (something that has never happened before).
4. They lucked out on the timing of the Tara Reade story (and/or were able to delay it long enough).
5. They lucked out on the timing of the pandemic.
6. The centrist competition was willing to drop out because (a) none of them were having any sustained success and (b) they likely view Trump as soft competition (i.e., you're not going to drop out in exchange for a Biden administration job if you don't think there will be a Biden administration).
7. Bernie was popular, but not so popular that he could power through this type of ratfucking.
8. Bernie was a good candidate, but he had a number of obvious weaknesses (he's an old white guy who had a recent health scare, he's from a tiny state, he's not the most eloquent or charismatic candidate, he had to break the ice on a century of anti-communist propaganda, he arguably was too nice, etc.).

It's also likely that Obama/Biden promised the VP spot to multiple candidates. Yang said as much about himself, and if Obama/Biden told that to a candidate as fringe as Yang, they probably dangled that in front of at least one other semi-popular candidate as well. That will be an open secret among the lanyard class who works on all of these campaigns, and future candidates are going to be more wary (because Biden pulled the rug out from under all of them by picking Kamala Harris). That goes double if Biden loses and those candidates dropped out and didn't even get a cabinet spot in return.

If any of the above plays out differently in the future (and you can substitute some other scandal for Tara Reade and some other disaster for the pandemic) we might not see the same result. They can't just flip a switch and shut out the left; they have to try, it's getting harder, and they won't always win.