That’s not what is happening though; it isn’t like 700 oligarchs self-appoint to the list of supers. For example, in my state in 2016 all but two of the supers were from democratically-elected offices, the other two were from the state party hierarchy selected and voted on at the state convention. So, not pure democracy, but representative democracy just like we use for most things.
The supers represent one way to deal with the reality that there might not be a candidate with a majority of the won delegates. Other ways, like going with the plurality winner or some form of ranked choice or condorcet or transferable vote, might or might not be preferable depending on what the ultimate objective is, but plurality is especially susceptible to stupid outcomes. If the objective is to find a candidate that reflects the global party members’ will reasonably well, then it is probably best to go with some kind of human intervention. The transition from back-room deals to pure democracy 50 years ago turned out to be an operational disaster, which is why the Hunt committee came up with the supers in the 80s. Human nature being what it is, I think it is better to have a publicly-accountable large group like that than a small or secret group.
Democrats who don’t like the selection process can always try to change it, and in fact it changes pretty much every cycle due to grassroots pressure. That’s pretty remarkable for a big political organization!
Everyone moans about the supers subverting the will of the people, but that hasn’t yet happened, ever. Even the notorious last cycle, when the DNC was trying hard to abuse their authority, the nominee was (unfortunately) the clear choice of the majority of primary voters. It isn’t worth agonizing over until it happens.
Of course, people who don’t like the process and don’t like the final candidate are under no obligation to vote for them. That’s true whether you think the nominee is an oligarch and you think that’s bad, or a Socialist and you think that’s bad. (Though last cycle the people who griped about the Democratic primaries then voted as a result for self-nominee Jill Stein maybe didn’t think things through very well.)
However, the chances are that, whoever the nominee will be, will be there because they reasonably well represent the party as a whole; the supers can’t give the nomination to a Tulsi Gabbard or a Michael Bennett (and wouldn’t want to).
And, even in the unlikely event the system screws up royally and Michael Fucking Bloomberg wins, while he might not be an infinitely better human being than Trump he would for sure be an infinitely better POTUS.