1. This will actually simplify the code for calculating the stack size. Instead of supporting special cases for jumping instructions in stackdepth_walk() you could just write something like

new_depth = depth + PyCompile_OpcodeStackEffectEx(opcode, oparg, 0);
if (new_depth > maxdepth)
    maxdepth = new_depth;
if (isjump) {
    target_depth = depth + PyCompile_OpcodeStackEffectEx(opcode, oparg, 1);
    maxdepth = stackdepth_walk(c, instr->i_target, target_depth, maxdepth);
}
depth = new_depth;

After adding new opcodes or changing existing opcodes you would need to change the code only in one place.

2. This will simplify third-party compilers (like https://github.com/vstinner/bytecode). They wouldn't need to duplicate the complicated code.