I've become increasingly convinced that QM is incomplete for various reasons, and so I no longer have confidence that coherence will scale the way we expect. I think it will hit a hard limit that we can't yet see because of this incompleteness. Maybe that wall will become clear once we have quantum gravity, maybe some successor to QM like [1] will clarify what's happening.
[1] https://arxiv.org/pdf/2102.07795.pdf