They could have added all of the above without breaking backward compatibility. This would have encouraged programmers to check for the enhanced features and use them if available.

Of course, long term, they would have needed to go with the 65816 and ultimately switch to another processor architecture.

I think if all of the 8-bit and 16-bit systems had survived until today, they would all be running on Wintel hardware but with their own operating systems, just like Apple Macs.