In the short term the pilot shortage will be felt only at the regional level. Long term it will hit the legacy's as well. Here's how/why:
1. The legacy carriers squeeze their regional partners so much that they can barely make a profit, if that.
2. Because of this squeezing the regional pay is low and QOL is poor.
3. Due to item 2, people stop learning to fly and stop pursuing airline careers.
4. Hiring at the legacy's pulls pilots from the regionals and the regionals are unable to fill the vacancy's.
5. Regional airlines continue to exist but shrink dramatically. I see them flying fewer but larger aircraft i.e. CRJ900/E-175. Staffed mainly by senior pilots who due to age etc. choose to stay at their airline rather than start over at a legacy.
6. Eventually, legacy carriers will have hired all the pilots they can from the regionals but still need more. Problem will be there aren't any more. They have exhausted that pool of pilots, see item 3.
At this point I don't know what will happen but it could be any of a number of things.
A. Legacy's will buy their regional partners in order to obtain the pilots.
B. Another raise in the retirement age.
C. Ab-initio training programs.
D. Single pilot flight decks with the right seat filled by a person trained to run flows, check lists, QRH, and talk on the radio, etc.
E. All airlines legacy and regional alike provide the same amount of lift but with fewer pilots by serving cities with larger aircraft and less frequency.