Very well articulated. IMO, this is very similar to challenges faced by microprocessor companies when introducing or modifying architectural features. The underlying micro-architecture can/will change to improve performance/power but existing architectural interfaces/semantics will remain the same or are enhanced with new features via new opcodes.
Backward-compatibility and Interoperability are social contracts with developers/users. This may be viewed as legacy baggage but guarantees that code running on one processor will have the same behaviour on any future processor.