Seniors are not entitled to playing time neither is the new freshman. Why would you bring up a Senior to Senior night if they are not part of that team???? That's pretty insulting to the self esteem of that kid. The enjoyment in HS is that you are part of a team that you can enjoy with your classmates, the people that you see and live most of the day. The growth in HS is when you understand your place within teams and learn how to be supportive and be ready when call upon.
Seniors in their last summer are trying to sort out the beginning of their adulthood. College, life is coming at them, so soccer is kind of the last thing in their mind. If you as a coach don't know what type of players your seniors or any returning player is, you might need to reassess yourself and your evaluation process. Older players don't change that much.
You might not accept it, but you implied it very well. And don't worry, lots of people are doing it, because that's their only source of income...
Parents can complaint all they want, but that's the beauty of HS Sports, parents should NOT have the same control they do in club. I laugh every time I see these helicopter parents complaining to coaches about playing time, practice time and what not, when in a few years they will be adult and they didn't learn, how to become self sufficient.
Disclaimer, I'm not talking about the Elite players nor the underachievers that go varsity sports to avoid PE and what not... Just to keep that in context.