I think the general feeling of disenfranchisement comes from the DNC influence in primaries. Who they throw their weight behind in terms of advertising dollars and publicity makes a big difference in how the primaries go. Also very relevant is which states have their primaries when, as most times the primaries are called before every state has theirs. This is also hearsay and way harder to substantiate, but there are also some that cry foul that there isn't actually much in place to monitor primaries like there are in standard elections and there is significantly higher potential for fudged numbers and misrepresented results.
Of course I would still love to see more left candidates get the nomination despite that, but it is certainly still possible that I am in the minority in that desire.
For 2024, though, it doesn't much matter if Biden is who the left wants. When running against a literal fascist, there isn't really as much room to voice complaints about the alternatives. I voted for Biden in 2020 and will again in 2024 even though he is much further right than my political leaning, and even for those of us on the left who are not happy with how Biden has been doing, there isn't really room to show that with voting because the alternative is so much worse. In this scenario, votes for Biden can easily be misconstrued as support for Biden which is not really the case