I think any projects that are run by people that see themselves as "X-people" (like Python-people, Perl-people) always have a bit "ick" reaction to new languages being added to projects they might see as part of a language's community.

So say you're a C++ developer, contributed to APT over the years, see all of it linked to the C++ community which you are part of too, and someone wants to start migrating parts of it to Rust/$NewLang. I think it might sometimes affect more for these people than just the code, might even be "attacking" (strong word perhaps) their sense of identity, for better or worse.