Regarding overhead bin space, I think carriers and their onboard staff can adapt by using some of the overhead space in the front rows of economy class as needed, by handling PE passengers’ luggage for them, if this means added margins. The staff already partakes in similar activities when overhead space is limited and/or the passenger needs physical assistance.
Given trends in economy class and how carry-on suitcase traffic jams are at times causing flight delays, I can see a future where carryon suitcases are no longer permitted for economy class passengers. We’re already on this slippery slope.
Yes, that is one possible solution. One of the overhead lockers makers is already demoing a locker with weight sensors and colored lights intended to be part of a reservation system, where premium passengers or those paying a fee would get reserved space in specific locker locations.
Of course, enforcement is hard. Airlines moving toward "basic economy" without access to lockers is becoming a thing -- and this can be enforced by making them board last and in a separate group, so the agents can check for big/multiple bags, and if a "basic" passenger does manage to cheat and find space, since they're last, they're not taking the space from a "paying customer"
Airlines are moving toward an increasingly price-continuity model, where you can buy a ticket at whatever price feels acceptable to you, and there will always be another enhancement of upgrade available at a modest cost, pushing you to spend more to feel less discomfort.
Given trends in economy class and how carry-on suitcase traffic jams are at times causing flight delays, I can see a future where carryon suitcases are no longer permitted for economy class passengers. We’re already on this slippery slope.