The danger isn't in the dem's leaving a vacuum if the nom is "stolen" from Bernie, the party will continue on it's establishment neoliberal path replacing all progressive dissent as fast as possible.  The real problem for the party is they risk a certain percentage of the next generation of voters "checking out" of the political conversation.  Enough Millenials and Zoomers will feel like the system is so rigged their vote doesn't matter (and they would be right) and will not waste their time and energy voting for a while.  It won't necessarily be all or even a majority but even disenfranchising 10% of them into non voters is the problem.  The GOP has the electoral advantage, if the dems push enough new voters out of the game they could start to lose the states they have been making in roads into.  The risk is that the dems make the swing states no longer swing states by pushing out only a small amount of a generation.  Pushing a state like Florida that is always a 1-2% state either way red by 5%.  Instead of closing the gap in Texas they push it another 5-10% back to red.  These problems will take several cycles to gain back the road they lost in 1 cycle.

Clearly stealing the nom won't ensure Republican wins everything for ever, only a GOP dictator could ensure that happens.  And of course we can't predict the future, but I think a reasonable argument can be made that it will ensure a second trump term and make it much easier for a GOP POTUS in 2024 and if that person isn't a 1 term president than that puts the GOP in control for the next 12 years (half a generation).

If the dems want to risk increasing the odds the will have to wait until 2032 all to stop Bernie now then they should steal the nom.  If they want a chance in 2020 and beyond they should let the candidate with the most delegates be the nomination