Let me try to be a bit more to the point, game engine features are not just "drop-in", by adding a feature to one part of the engine there's a high probability that you make another part of the engine worse.
There's a reason that they say performance is the most leaky abstraction.
Then that sounds like you would want it to be an optional plugin, or a compile time flag that could be enabled for customers who want it? Why not do it, if that's what the customers asked for?
I get what you are saying and it definitely applies to the core architecture of the product, but if you have a large number of customers each with their own needs then I would be surprised if there were zero parts of the product that were modular or interchangeable.
> then I would be surprised if there were zero parts of the product that were modular or interchangeable.
You'd be surprised. Game engines are extremely monolithic.
There are some modular components and techniques (eg pathfinding, rendering, physics engine), but most of the effort that goes into an engine like Godot is integrating these components to their specific architecture. While the component is transferable (eg you can always use the Bullet engine in your own project), the integration work isn't.
There's a reason that they say performance is the most leaky abstraction.