Your description still has problems on architectures like x86 with instructions of different lengths. In particular, you pretty much have to use a single-byte

Your description still has problems on architectures like x86 with instructions of different lengths. In particular, you pretty much have to use a single-byte instruction to do the trap; a multi-byte one runs the risk of clobbering other code that could well be jumped to. That’s why INT3 exists as a single-byte instruction to begin with!