@LBG: It was just a quick idea from me (and maybe wrong, of course) as for seeing people demanding always backward compatibility with their already existing and known, old things.
It looks like a good idea in this respect to have those improvements on C64 mode for instance (to use the 6510 instruction set + original timing + VIC badlines + etc. á la DTV).
It might be a similarly good idea (or might not) to also preserve the original C65 behaviour as it is now for the future (only for the "safety" in a convenient way) along with the new one.
Just consider that something like the "mono check" for sound design & audio mixing.