It won't do much though. Although it will make things a bit easier. A lot of minecraft mod requires direct modification to Minecraft's core mechanism. For example in popular mods: create, immersive portal and distant horizon all taps in minecraft's collision mechanism or rendering pipeline in a quite aggressive way that unlikely there will ever an abstraction be able to handle that.

And, no. Although forge does not encourage you to use it. It also supports mixin like direct modification. It's just less polished than mixin(requires more knowledge to java Bytecode)

It's unrealistic to ask mojang to make an abstraction that you can actually change everything in the core without direct modification (let alone how do mojang know what part need to be abstracted?)