It is important to distinguish between "Does the flow work" vs. "Will the flow be there when there aren't enough new regional pilots to fill the seats of those people flying at a regional today"? By all rights this is a pyramid scheme.
Let's break this down into 3 answers as they relate to American. What happens at Delta I have no idea.
Does the flow work? Yes, people who are flowing today are doing so without having to formally interview with mainline.
How long till you flow? This is a touchy area, especially if you read these forums. For a company like envoy, it looks as if the actual pilots that are flowing today have been with envoy for 16 years. That's a long time. Now, people from envoy will tell you that the future of the flow is a shorter period like 5 years. Lots of debate here and many aren't buying it.
This leads me to question 3- Will the flow be there in the future? I don't see how it can be. Because of the pilot shortage, in order to continue the flow in the future, American would have to destroy their own regional feed in order to keep their promise to flow new regional pilots to American. Do you think American would destroy its own regional feed to keep the flow going? I don't think so personally. I think one of two things are much more likely-
1.) American decides to destroy someone else's regional feed- ie hire pilots from non-wholly owned regionals such as Republic etc..
-or-
2.) American decides to add a new rung to the career ladder by saying anyone who wants to work at American MUST go through one of their wholly owned regionals no exceptions. That would mean that a pilot working at a carrier such as Republic would have to quit Republic and go to work for PSA, Envoy, or PDT.
EDIT: Add new option
3.) The regionals are simply absorbed into mainline. The only thing that separates us today is payscales. Other than that, its the same job.
Just my .02.
But think about it, if you are American, do you destroy your own feed just to keep your word on flow with the pilots? American has a history of not keeping their word
One thing to keep in mind, 6 years from today more major airline pilots will have retired then there are regional pilots to replace them with.
Let the sh*t storm begin !