I know how you feel. My mom (also 81) does Pilates. It saved her life (and I mean that quite literally . . . she would be dead by now without the motivation and, yes, the physical strength and health it brought her). Instead, she's a fairly active, and still quite independent lady in LA, still living on her own, and likes it that way.
Pilates isn't CrossFit, but you gotta start where people actually are, and with what they will actually do, not with where you wish they were or would do.
I don't think Pilates, or Curves, or even "aquacise" (water aerobics) is the best choice of exercise for someone who is not genuinely frail. But I'm still happy to see people doing those things rather than nothing. Maybe 1 or 2% will discover a yearning for something better . . .