I mean, they have a lot to lose. There are strong alternatives. Unreal and Godot are at the doorstep. Godot doesn't take anything at all, Unreal takes, but in a reasonable manner and it's of course on 3D a lot more powerful and also offers an asset store.
The games already developed and deep into development are unlikely to jump, but future games will have a huge argument against Unity now. Unreal could completely snap their necks now by putting into writing that they never do such move.