It's one of the difficulties of the game design. A new type to a RW airline means a bunch of paperwork, retraining of Maintenance Staff, Cabin Crew, and Pilots. That can include new tarmac equipment, trolleys, push back equipment etc and the facilities to store and maintain it all. The Aircraft Maintenance department takes a big hit too with Lead Technicians (Engineers design, Technicians do in my world) being flown to Boeing for training together with all the miscellaneous stores stuff from new tyre types upwards.
I think the game misses the impact of one new type, and there should then be a lower cost as the numbers of that type rises.
I'm not aware of Boeing types being considered totally different, a 727 has the same sort of paperwork as a 757 even if the parts are quite different.
What we don't have is 'wet lease' options where the lease does all the C and D servicing at a maintenance base. It's what some airlines use to cover the longer absences of their aircraft for the D checks.
Wishfull thinking, one day, lets have the cargo stuff first. That's going to be a whole new game in it's own right.