Caucausing isn’t really comparable to coalitions in my opinion, because all the formalisms are missing.
Bernie Sanders has no actual power within the party, no matter how many people voted Democrats because of him.
Compare the situation to an actual multi-party system with coalitions. Sanders would have his own party and there would be 1-3 other parties that are currently part of the Democratic party. Each of these parties would collect separate vote shares which would lead to some of these parties being larger and others smaller. Voters would have to choice to express which exact political direction they prefer instead of just having a binary choice.
After the election, coalitions would be formed. These coalitions wouldn’t have to be along the current party lines, but e.g. moderate republicans and moderate democrats could form a coalition with eachother. This way, coalition-based multi-party systems tend towards moderate compromises, while two-party systems tend towards extremism.
In a multi-party system centrists represent reason and compromise, whereas in a two-party system they represent boring blandness.
In a coalition, each of the coalition partners hold power, because everyone of them can end the coalition. This means, more compromise is necessary and someone like Sanders cannot just be ignored for decades.
It’s not a form of censorship, it’s a form of democracy.
If you are not ok with a downvote reducing visibility, then by extension you should hate upvotes just as much, since they reduce the visibility of everything else.