I've lived through the 16 to 32 bit and 32 to 64 bit computing transitions, so I'm base sceptical that we've maxed out (though I wouldn't rule it out). Definitely there are cryptographic primitives which want larger numbers.

The real problem is handling overflows: it's actually simpler to say "we don't do that" with some certainty (you *can* actually hit size limits in GSR due to stack size limits, but 4MB is a fairly large number!).

TBH: The complexity for people *writing* Script dealing with signed numbers is actually a much bigger problem than the well-known issues of simple arithmetic operations on large numbers in the implementation.