I seriously don't get it though. Rust is a nice language, but so is X. However we don't see X people brigading existing projects with constant bombardment with "rewritten in X". What is that about Rust that prompts this behavior?
People see the safety as a moral superiority so it attracts obnoxious zealots.
Other languages' features and syntax aren't nearly so easy for zealots to form behind. The perception of absolute safety it puts in some people makes them crazy.
People were told for years they can't use Rust for their new projects because it hasn't been "proven" in industry yet. So the option was to sit back and wait (chicken and egg) or move to rewrite a bunch of projects so that it could actually be "proven". Not saying this is the only reason why it happens (every language has its Zealots) but it certainly makes more sense.
Due to the explosion of new programming languages over the past few decades your options are to either aggressively expand wherever possible or die out because you're not "proven".