The Mid-Am champion does get a Masters invite, so it's not solely the pre-professionals -- but yes, they are the vast majority of amateurs at the Masters/US Open/Open Championship.
I think most mid-am winners lately have been "renistated" amateurs.
Jeff,
I think you're mistaken. While a large number of the players in the Mid-Am are reinstated most of the winners have not been.
Going back a to 2002:
2002 - George Zahringer - I don't believe he was ever a pro
2003, 2009, 2010, 2012 - Nathan Smith - Definitely not
2004 - Austin Eaton III - Again, I don't believe he was
2005 - Kevin Marsh - Reinstated
2006 - Dave Womack - I don't think so
2007 - Trip Kuehne - Definitely not
2008 - Steve Wilson - He was reinstated
2011 - Randal Lewis - definitely not - per David Cronin
2013 - Michael McCoy - Yes, but only because he went to Q-School out of college when that just going made you a pro. He has said he wouldn't haven't turned pro if he wasn't required to do so back then.
2014 - Scott Harvey - Reinstated
2015 - Sammy Schmitz - Reinstated
2016, 2021 - Stewart Hagestad - Definitely not
2017 - Matt Parziale - Reinstated
2018 - Kevin O'Connell - Reinstated
2019 - Lukas Michel - Definitely Not
2020 - No championship
2022 - Matthew McClean - Definitely Not
So, since 2002 there have been more players who never turned pro than did. Of those that did they really only very short runs as pros and none every came close to any success as pros. I don't know why this should be such a controversial thing. Every person should be entitled to explore their options.
Over the past 10 years, the USGA setup very specific requirements on how long players have to wait based on the number of events, cuts and other levels of success that players have had on professional tours. Some of these have resulted in players having much longer waiting periods than in the past.