Fitness+ comes with one of the tiers of Apple One. That makes it "free" to the user. Let's say you only wanted Music, TV+, and the storage (especially useful if you have multiple iDevices in your family and you want cloud backup):
Music - $9.99 individual, or $14.99 family
TV+ - $4.99 (already at $20 for a family here)
2TB Storage - $9.99 (now we're at $30)
So Fitness+, News+, and Arcade are "free" additions if you get Apple One Premier. Of course, you need an Apple Watch to use Fitness+ but you can use it with any garage sale treadmill, bike, rowing machine, yoga mat, dumbbell set, etc.
And if you have the Apple Watch you've already been able to drop things like Runkeeper and Strava for tracking running and cycling.
apple has historically targeted the mass premium customer segment, able to command a higher price and still entice the broader public to buy. fitness+ (and apple one) has the higher price but not the broader appeal. it's just not differentiated enough.
a personal anecdote: i've had apple tv+ for free for the past year and have watched it maybe a half dozen times. the shows are so overproduced to be tone-deaf. they're now trying to buy their way into the market via awards, so we'll see if that turns into any compelling content and moreover, more subscriptions.
apple tv+ is just not robust or differentiated enough from netflix, hbo, disney+, or anything else to be a thing. disney+ goes all-in on family, while hbo goes all-in on adult. netflix has the all-you-can-eat buffet model where you'll be comforted to find something, anything, to watch in a pinch. apple tv+ has nothing like that. there's no brand or identity to form a memorable market segment around.
That's where they end up making money anyways, 75% or so of their revenue is from device sales (computers, phones, tablets, watches, etc.). The services help to keep people within that hardware ecosystem which gets people onto the upgrade cycle or to consider a second or third Apple device instead of just having one.
first, the market opportunity is much smaller. everyone listens to music (and has a phone nowadays). that's a huge reachable market. only the well-off will pay for fitness+, so you're looking at <20% of everyone to start.
second, music was already in the transition from physical (cd's) to digital (mp3's) to streaming (just waiting for high-speed internet penetration). the fitness market is not in that same transition. it's still constrained by the physical world, no matter how 'social' it gets.
third, peloton's hardware is what makes the ecosystem, not the other way around (which is apple's strategy). it creates a halo of influence around the purchaser through price and product that apple simply can't replicate with just an apple tv, a watch, and a subscription. (i don't like peloton personally, but the market dynamics here are recognizable)
i'd be surprised if fitness+ really takes off. it has narrow appeal, a relatively high price, and little cachet or differentiation. zooming out, apple's "plus" strategy overall strays away from their strength in hardware-based consumer products, and i'm skeptical they can differentiate enough to retain that business long term, whereas homekit, carkit, and the like couch very neatly into that strength, and likely represents durable future expansion.