I wasn’t suggesting that the parents would be denied Plus Loans. The student was talking about co-signed loans, that would be HIS (or her) responsibility.
Plus Loans are parent loans that the PARENTS are responsible for paying back. The student has no obligation at all to pay back Plus loans.
Open the 529 account. or give the money to your parents for them to pay your tuition with.
But…as to your question about how do middle class parents afford X schools…well, most can’t. Most American students commute to a local school because their families cannot afford to pay very much for college. Most students do not go away to college.