Problem was that the S'ona were dying
Probably becuase they were using crappy non-federation medical tech, just switch over to federation medical technology. It kept McCoy alive so it should keep them alive long enough to figure out how to make their own magic youth radiation.
Besides that... the Son'a were aging. Big deal.
150 year old McCoy, would he force the Ba'ku of the planet just because he doesn't want to get old like everybody else?