I slightly disagree. Introducing a new multibyte opcode is IMO not a huge problem. However, EIP-615 included a lot of assumptions about code, such that a runtime jump analysis should not be needed (because of deploy time validation), or a double-pass of a much more complicated analysis would have to be made during runtime.
If we were to introduce e.g. JUMPSUB 0x00ff (defining it as in total three bytes), I don’t see any huge problems with that … ?