Ouch! Tough audience...
The A321 rate was a shortsighted mistake made years ago.
You've even answered your own issue. Everything's pretty easy in hindsight. The reasoning was sound, the "family" rate at the time was reasonable (better than, regionally), and arguably the NEO is a new type altogether...
With the 787 however, the -10 (currently) has the same MTOW as the -9.
and negotiate a new method to calculate fleet pay
Good thing the pay-rates are based on a payload/range equation and not just weight I guess. 40 more seats, 15% more cargo volume...
The same problem will manifest itself when the 777 3’s are replaced (777-900 similar gross weight etc).
It's unlikely a 330 seat aeroplane will be replaced with a 440 seat one in any case (that, and a 777-
900 doesn't exist), so more likely a 777-8 is the machine. Again, payload to the rescue...
Looks like there'll be plenty of volunteers for ALPA negotiations, and that can only be a good thing.