Let's say that every single one of them votes for Biden in their respective caucus/primary *but* it becomes clear next summer that Biden is physically unable to run for President.

Do you think a single one of these people would hesitate to say, "Please run Gavin Newsom" in that case, if it was obvious that Trump was going to otherwise win?

It doesn't matter. If you're in an early primary state (like New Hampshire), and you cast your ballot for one candidate, and later in the primary cycle you wish a different candidate had won your state's primary, you don't get to go back and change your vote. You don't get to "undo" their primary victory, or their allocation of delegates.

These processes are set up to render final decisions. Once the votes are cast, it's done. The delegates are bound to those votes.

Again, it's not like the old system. The old system had nonbinding primaries, that existed just to provide information to the delegates. The voting was what happened on the convention floor - not in the primaries. That's not the system any more. The delegates don't pick the winner - the winner is picked in the primary elections (and caucuses).