To be fair, while the Dems having selected their candidates in the last three elections as they have done is entirely legal, it has been pretty underhanded and, if you ask me, not very wise, either.
The thing that Trump understood that the Republicans in 2016 and the Democrats from then through to now do not seem to get is that business as usual in Washington has got to go. The actions and pointed inactions of the federal government since the Reagan era have been slowly strangling the middle class, and folks are getting tired of working two jobs plus a side hustle to just stay above water—that’s not even building a savings or accumulating appreciating assets. Hard work does not get a person ahead any longer, and it’s starting to be the case that hard work doesn’t even keep a person in the black. And yet, the Dems keep using super delegates and behind the scenes shenanigans to pick safe, business-as-usual candidates, and when they do get into office, even the candidates that campaign on change just perpetuate the same old policies that got us here (Obama protecting the banks and wealthy that had caused the 2008 crisis, Biden not pushing to expand the Supreme Court or make DC and Puerto Rico a state, the ACA still relies on for profit insurance…the list goes on).
The Dems had a shot in 2016 to run someone who would win, but they blew it on grounds that he would be seen as too far left (which is not what would have happened at the time) and at this point, the right has taken over the narrative and set the frame, precisely because the Dems kept running business-as-usual candidates. As much as I disagree with MAGA and Trump, I do understand their movement is built on real grievances that the Democratic Party has simply ignored for the last 30 years, and seem to want to continue to ignore. So, yes, what the Dems have done is all legal, other actions would also have been legal and would have put us in a far better place right now as a country.