Aside from the fact that memory allocators are written, rewritten, and tweaked all the time, your own organization's actions defang this argument: Firefox 3 included a switch to jemalloc.
A version of jemalloc specifically written for mozilla's codebase.
If it were a 100% compatible drop in replacement for malloc, why hasn't every other project switched to jemalloc yet?
(Searching for switching to jemalloc, seems like every group that tried it out found that their code base or use case exposed new problems in jemalloc. That these could be fixed doesn't negate the main point -- you couldn't just switch malloc out in existing codebases and expect everything to work right.)
I don't think I've ever seen a more dramatic real-world example of unwittingly strengthening the point you're arguing against. jemalloc didn't work quite as they wanted, so they customized it, and you really think that's a counterpoint to an argument for trying new and different things?
Ah, you're right, it's so simple! If changing the JS engine breaks a web site, the sites authors will either customize the JS engine, or choose not to switch after all!
Aside from the fact that memory allocators are written, rewritten, and tweaked all the time, your own organization's actions defang this argument: Firefox 3 included a switch to jemalloc.