Because LoCo for life is just not sustainable. Health wise.
True. The LoCo's will need more pilots as many of the older more financially secure ones will go part-time. They will demand 50-80% rosters; hence more pilots. LoCo's have relied upon self-funded cadets; that may change as funding dries up. They also benefited from qualified pilots being on the market as they drove less flexible airlines to the wall. So they benefit from greatly reduced training costs from the old traditional model.
The major more traditional airlines are also expanding and looking to cut costs. They seem attractive career moves for those near burn-out in LoCo's. There is a belief that those guys are well trained and well experienced, so easy to recruit into the long-haul/short-haul operators. The majors now have a source of ready trained pilots at reduced training costs. It will depend how soon the change their philosophy of preferring home grown talent from zero in their own flight schools. Can they teach old dogs new tricks. Will it be a sound idea to have a LoCo captain as a 2nd officer in a seniority system? This will further put a load on LoCo's recruiting numbers.
It is very difficult to predict what the pilot market will be in 20 years; indeed what a life-long career will look like. Will sacrosanct seniority still rule in the traditional majors? 30 years ago pilot mobility was very limited. Look at it now. There are many various options to switch employers; sometimes with little or no backward step.
I think the LOCo's need to step into the homegrown training market and accept that many pilots will stay with them for max 10 years. The majors might need to change their recruitment strategies. Time will tell and it will be the bean counters who decide.