This is a valid point. I've played a little with Bevy and liked it. I have also not written a triple-A game in Rust, with any engine, but I'm extrapolating the mess that might show up once you have to start using lots of other libraries; Bevy isn't really a batteries-included engine so this probably becomes necessary. Doubly so if e.g. you generate bindings to the C++ physics library you've already licensed and work with.

These are all solvable problems, but in reality, it's very hard to write a good business case for being the one to solve them. Most of the cost accrues to you and most of the benefit to the commons. Unless a corporate actor decides to write a major new engine in Rust or use Bevy as the base for the same, or unless a whole lot of indie devs and part-time hackers arduously work all this out, it's not worth the trouble if you're approaching it from the perspective of a studio with severe limitations on both funding and time.