American already has several slots into LHR. I counted 24 arrivals into LHR tomorrow.

One 777 all economy plus one Boom aircraft all business is roughly equal in seat miles to 2x 777 (first + business + premium economy + economy).

An upstart airline would need to acquire a slot to get a new aircraft into LHR. However American could add a super sonic aircraft to its service and still maintain the number of the seats it has going in. Yes, it would have to take one aircraft out, but there are enough pax that will be connecting and can be sold a different routing that it can use flexibility with seat configuration to not be limited by slots.

For American, a bigger factor is probably what kind of seats they can sell. For instance, DFW->LHR has a 75% load factor tomorrow for first class but a 68% load factor for economy class. However, later in the week economy has a higher load factor.

Those first class seats are a lot more profitable than an economy. Adding first class seats while taking away economy seats is profitable if you can fill the first seats.