My worry is that this will mean lower interchange fees paid by merchants accepting M/C but not Diners.

Those fees fund Club Rewards, and Diners Club could well want to cut costs in their rewards program.

Now, I don't KNOW that lower fees are involved. I suppose it's POSSIBLE that Diners Club cards will yield the higher interchange fee, even to Mastercard merchants, but I find that a bit unlikely (forcing all MC merchants to pay a higher fee than they had signed up for).

Worrying about lower fees and what that will do to my points...