That's the problem with your argument.

I know many regional captains that wouldn't leave even if they offered a pay raise to their current pay. The pay, benefits, and seniority is too good to go back to sitting reserve even if it's a short period.

There are many corporate pilots who would NEVER leave for a major. They get weeks and holidays off and similar pay. There are many 91 pilots that fly only a few hundred hours a year. Why would they all of a sudden Want to fly almost 700-800 for a slight increase in pay.

Yes. Some would leave but to say all ATPs are major airline candidates oversimplifies the conversation. Also, major airline flying isn't the entire industry. It is composed of cargo, airlines, corporate, on-demand, charter, and private flying. If one of those sectors has a hard time attracting applicants, which I believe they will in the heat of major retirements, it is a shortage. Period.