> The issue is linear evolution means the measurement of a superposition leads to a superposition of measurement devices. If the quantum state is real that gives you many worlds.

And that's problematic because? Because it explains away the whole measurement problem, there is nothing to explain, it's an artifact of a macroscopic observer's point of view?

It's like saying that SR/GR with their space-time continuum being real is problematic, so let's keep to the (post)-Newtonian point of view, but oh no, it now has all this weird amendments and additional terms, and when you try to extend it to the whole of the Universe, it breaks down/gives really weird stuff. Well, duh, of course it does, if one tries to pull an owl on a globe, it simply won't fit.

Well you don't need to have nonlinear evolution to get what alpineidyll3 is saying. It's sufficient for the observable algebra of macroobservables to be commutative. This allows the evolution to be linear and have no interferences.

The QM is "an island in theoryspace" idea isn't strictly true either. QM is one among an entire family of probability theories. It's only rigid when considered purely from the point of view of Probability theories based around vectors in Hilbert space. However considered as part of OPTs in general there's nothing that makes it difficult to modify.