That may or may not necessarily be the case.

With LHR being slot controlled, even if it were doing poorly, they may toss a small aircraft in the mix to retain the slot. That could be why we see so many 752s.

Without knowing the internal numbers, we don't know if this route is doing well or not.

Another example of this sort of behavior is UA flying LHR-BRU with a 777 just a slot placeholder.

There are quite a few non-obvious items that go into scheduling. In some airports, they may have gate turn requirements in order to keep gate leases they want to preserve, so sometimes that will factor into the schedule.