18m > 19m = 6% increase.
RYR say 737-8200 is 4% larger capacity than their 737-8s.
WUK A320CEO to A321NEO is 21% increase in seat capacity but I think they only have 2 based now, others already are 321NEO.
So as you say, probably only room for 1 more overnighter. Bids for slots form an orderly queue.
If 19m gets approved, how quickly will a small jump to say 21m be asked on the grounds of encouraging quiter more full efficient planes which by design are larger?