>It's why places like Planet Fitness are able to offer $10/month unlimited memberships.

That model is far different though

For each Planet Fitness, much of the cost is upfront. That is, they have to buy the property, renovate (or build), and then buy all the machines. Yes there are increased maintenance costs as more people join the gym, but cost per user should decrease with each additional membership.

For MoviePass, for each user they have to pay for the price of each movie ticket they go to. Thus their costs increase proportionally to the number of subscribers. Unless each additional user they sign up goes to the movies less than the average MoviePass user, Cost per user doesn't decrease.