I'm sure my suggestion would not apply to a route such as EDI or FRA, but some destinations seem overserved by too-small aircraft. 198 flights a week to New York using relatively small a/c isn't a good use of scarce slots.
But even the New York example doesn't stand up to examination.
The only narrow-body aircraft used on that route are 757s, as you rightly say, but hardly "3 every hour". In fact there are only 2 757 flights
a day (3 on Sundays and Mondays), and those are to Newark (all JFK flights are wide-bodies).
So consolidating those, even if feasible, would hardly "free a fair number of slots".