But they could only do that for the office of president+VP, a national office. They don't have conventions for choosing candidates for any of the other offices--those are decided in the primary, period.

Also because of the backlash from thatm after similar issues in previous years, there was some reform to reduce the power of the superdelegates:

Superdelegates would now be prohibited from voting on the first ballot at the National Convention if their votes could be influential in the outcome.4 (However, if their votes could not sway the nomination, superdelegates would be permitted to vote on the first ballot.) If no candidate received a majority of pledged delegates on the first ballot, superdelegates would be allowed to vote on subsequent ballots.

See https://pmc.ncbi.nlm.nih.gov/articles/PMC7786873/ for more details.