Not sure if it would have the desired effect. If regionals can only fly 70 seats or less can mainline only fly 70+ seats? If that happened you just ensured the survival of regional airlines and eliminated the ability of mainlines to bring that flying back in house. I dont think that will help. The ultimate goal, in my opinion, should be to get that flying back to the mainline and eliminate code share agreements.
If that happens, regionals would be forced to compete against the majors instead of being a parasite attached to their underbelly.
I think what you really have to do is either fully regulate the industry or completely cut off government funding to airlines. State governments are throwing millions at start ups, who then pay pilots pathetic wages and charge the public $10 per seat, undercutting every respectable airline. Larger airlines then need to slash prices, causing them to go even deeper into the red then go looking to congress for a handout to keep them afloat. Airlines exit bankruptcy leaner, employee compensation slashed again, and the cycle begins again.
At least that is my take as someone just starting his journey into this profession.
Taking the flying "back to the mainline" will never happen. At the very least, it's highly unlikely. Unless we're going to a mass-regulatory effort with "one airline, one ticket" type rules that's fairly far-fetched.
As far as limiting the regionals, well, why not? If we're going to call them regionals, let's keep them regionals. If major airlines want to do regional level flying, i.e., fly small planes, I see no reason why not. Why airline managers would let pilots fly little planes at big plane wages is beyond me.
The whole point here is to stop regional growth. If you don't, there will eventually be no mainline jobs left to go back to. Erasing the regionals as they are is too extreme- that sort of legislation would die an early death. Simply accepting that things have split already and creating boundaries for that is more realistic.
Eventually, if regional growth (and thereby, attrition of flying at major airline costs) is limited, it will be forced to survive on its own economics. Either 70 seat and less seat flying will become a part of the business structure that works, or it'll go away over time and fade out.
In other words, if you take away the scope-busting, major-airline eroding abilities of the regional feeders, you force airline managers to face something. They won't kill off what remains of worker quality of life from the Regulation era quite so easily.
That's what regional growth is really all about. It's about completing the final step of the Deregulation cycle- killing the 'regulation' of labor Contracts and breaking wages and quality of life down to their lowest possible levels.
If we don't pull the fangs of regional growth, the lifespan of the 'big time' jobs at majors is limited.. and likely short.