> you almost certainly get identical software out the other end

I think you'd get a set of convergent solutions, where the actual implementations might be very different.

This also explains why you might want to keep more artifacts than just the design. There might be different performance overall, or interesting properties/side effects on axes that were not defined in the spec.

Possibly -at the limit- a theoretically 'perfect' spec would be 100% deterministic (eg. by defining the final object code down to the bit) but that wouldn't be a useful operation I don't think?

The other approach to getting bit-perfect output would be to aim for deterministic model runs. But this has numerous issues including making the model less "creative" or "smart" and anyway might not be the thing you actually want (to wit: a working artifact) .