[comex]

Indeed, but this isn't hypothetical. A large fraction of multiplayer games operate by sending inputs and requiring the simulation to be deterministic.

(Some of those are single-platform or lack cross-play support, and thus only need consistency between different machines running the same build. That makes compiler optimizations less of an issue. However, some do support cross-play, and thus need consistency between different builds – using different compilers – of the same source code.)