> The reason gyms can offer flat-rate pricing is that they bet on under-utilization, not that costs are flat.

Yes. In fact i remember hearing about a gym which offered a flat-rate pricing model but explicitly excluded certain professions from partaking in it. I remember the deal was excluding police, bouncers, models, actors and air stewardesses. They had a separate more costly tier for these people. (And I think i heard about it from the indignation the deal has caused online.)