The post posits that, to handle the burden of legacy instructions, an "obvious technically-appealing approach (is) starting over with a clean-sheet architecture".

The approach that immediately occurred to me would be have a layer that translates the legacy instructions into modern equivalents; without as much concern if they are slower to execute in their new form (they're legacy, after all, right?).

Of course, doing something like that is probably nowhere near trivial, the devil's always in the details.

But I bet this is already being done at the microcode level. Stepping things up to having a published agreement about which instructions were globally considered "legacy", and guidelines for what their equivalents were, would go a long was towards allowing a general feeling that an ISA was evolving, rather than just accumulating weight upon weight.