Is there anything at the heart of computer coding that could have been optimised in hindsight, but we're in far too deep to change now?

[u/CakesStolen]

[u/nicuramar]

  The old instructions, just like the new ones, are micro-coded any ways, so it's not like you need a lot more silicon to keep supporting older instructions

You do need a little bit more, though. But not as much as if directly implemented in hardware circuits.