Allowing amount arithmetic for a limited range of values seems unergonomic in the best case (wallet devs have to write code to prevent overflows) and a footgun in the worst case (wallet devs don’t write correct code addressing overflows and money can be stolen by fee siphoning attacks or destroyed by accident).
Although I take no position on the desirability of these opcodes, if they are to be added, I would favor a design that either doesn’t allow arithmetic or which is compatible with a previously or simultaneously activated soft fork adding 64-bit operators.