It’s not just the scripts themselves. Writing a correct Script interpreter in this regime becomes more complicated too. Script interpreters have to match each other exactly if we don’t want consensus splits.
We should be moving towards formally specifying the Script VM and things like a hypothetical OP_ENABLE64BIT make that harder than just putting new 64bit opcodes into the VM. I do recognize there is a desire to not eat up extra opcode space, but I feel the need to state that introducing this new invisible interpreter altering semantic is probably a pandoras box we don’t want to open.