>not much of a reason for it to persist after that

for working out and keeping fit!... oh, hmmm, I guess that's glaring evidence that working out and keeping fit are not beneficial after all

I am clearly speaking from the perspective of evolutionary pressures. And you're right, there's very little evolutionary pressure on working out, staying fit, staying happy, etc. etc. beyond the bare minimum required to ensure propagation.

Once you can't reproduce and can't help with reproduction evolution doesn't care if you stay fit.

Well that's not the point of play to begin with, it's learning motor skills, so your logic doesn't make any sense.

You don’t need to play to stay fit.

Adult lions don’t play like baby lions yet adult lions are still fit.