Nonsense.

The gym is an investment in your health, one of the best investments you can ever make. 

 Lol wut? We live in a world of computer work and cities. Not everyone has the ability to go chop down a tree on their lunch break.

Even if fiat goes away, I'm assuming we'll still have the need for gyms 

 One small example of cities, use the stairs instead of the elevator, imagination is the limit.