The path to evolve for the 65816 while keeping backwards compatibility would probably end up as a convoluted mess of layers not unlike the x86.
The path for the 68000 was much clearer - it already starts from a clean 32-bit architecture with a 16-bit data bus and a 24-bit address bus. The 68000 and its offspring also powered multiple Unix workstations at the same period, creating a critical mass of development tools that didn't exist for the 65816. It went on to 32-bit data and address buses, FPU, MMU and extra instructions and survived for some time after Apple, IBM and Motorola mostly gave up on it and went on with the PowerPC. All in all, the 68K architecture carried the Mac line for more than 10 years.
That would be very unlikely to be possible with Apple II software compatibility. In this case, the PDS Apple II board was probably the best option for Apple. It could have been designed to have a version of GS/OS using the Mac an an IO device in a software-friendly way, but the GS/OS software base was not there to justify the extra effort.
The 65CE02 was a very slight improvement over the 65C02. It's a bit faster at the same clock and has some tricks that appeared on the 65816, but, apart from that, is not that impressive.
The C65 would have been a much nicer computer than the C128.
Unfortunately, for Apple II users, the fact it doesn't match 6502 timings perfectly makes it difficult to use as a drop-in replacement, as time-critical ops in joystick sensing and floppy IO may need some adjustment.
The Commodore 64 has a 65816 Super CPU add on that some games can take advantage of. It is needed for that Super Mario Brothers clone to get proper scrolling.
The path for the 68000 was much clearer - it already starts from a clean 32-bit architecture with a 16-bit data bus and a 24-bit address bus. The 68000 and its offspring also powered multiple Unix workstations at the same period, creating a critical mass of development tools that didn't exist for the 65816. It went on to 32-bit data and address buses, FPU, MMU and extra instructions and survived for some time after Apple, IBM and Motorola mostly gave up on it and went on with the PowerPC. All in all, the 68K architecture carried the Mac line for more than 10 years.
That would be very unlikely to be possible with Apple II software compatibility. In this case, the PDS Apple II board was probably the best option for Apple. It could have been designed to have a version of GS/OS using the Mac an an IO device in a software-friendly way, but the GS/OS software base was not there to justify the extra effort.