Nblhorn: I think there is a market for a GOOD fitness game with a one time purchase, and maybe DLCs as an additional source of income for you.
Similar model to games like Beat Saber, Puzzling Places, Synth Riders, where you can unlock additional tracks or levels for smaller one time payments.
People are tired of subscriptions and I would imagine the amount of people who willingly spend 10$ a month for yet another subscription will be pretty low.