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My answer to the post above from the other thread... it's probably more appropriate to discuss this here
Whatever happened to Terbium?(WDC's 32 bit 6502 evolution)
But your idea is interesting... 65816 was shown to be roughly comparable to early 68K Motorolas. It's probably much cheaper as well. But since Lorraine was based on 68000 it would mean building a whole new chipset basically. I do not think that's likely to have been approved.
But 65816 maybe should have been used in the 128 or a similar C= branded machine.