Any barn worth its salt takes into account the twilight years of their lesson herd, and provide accordingly. That should not be a long-term, life of the animal loss (the horse should bring in more over its lifetime/during working years than you spend maintaining it during its retirement, on average -- obviously sometimes you win, sometimes you lose) but on a year-to-year basis, the books should reflect a loss on that particular animal during its retirement.
Any barn that did not treat its seniors well is repugnant to me. And any barn that does not take this basic business principle into account when structuring its program is run by an idiot.
I have no problem with programs that anticipate this and sell their schoolies to good homes when they are in the 12-15 range, with several good riding years left.