It's a mixed approach in the end. Bevy is 100% ECS from what I gather. Fyrox is doing a pragmatic approach of taking what's good from different engines. I'm sure as it evolves it will abandon the things that don't work in a Rust context. So to rephrase: there is definitely a need for a game engine in Rust that's not 100% ECS.
> Isn't the answer quite obvious, which is that if someone wants to use an engine built off "proven old tech", they should probably just use one of the major players in the space
All of the major engines are written in C++. I've tried Unreal, Unity and Godot. They all have their flaws. With Unreal and Godot those flaws are mainly the scripting languages BP and GDScript which one can get tired of quite easily, especially BP. You can of course use C++ for all your scripting but both engines strongly encourage you to use their scripting solution for a big chunk of your game. (Unity has a bunch of other issues and isn't source available, although C# is fairly pleasant to work with IMO).
There is definitely need for an OOP engine in Rust just as there is need for an ECS engine in Rust.