It is true in some sense, we don't just implement things Servo wants. If Rust was beholden to Servo, we would have put inheritance in the language over a year ago, for example. That said, the eventual work for "something inheritance like" is absolutely predicated on Servo's needs. But we put it on the roadmap and weight it like anything else.

That said, we do take their needs into account rather highly, just like any project with a large upstream user.

Just like we wouldn't want Rust to be useful for only compilers, "only compilers and web rendering engines" isn't much better.