I don't know how widely spread this is, but even before the pandemic my company had a gym benefit and it can be applied to Peloton. I didn't know that going in though, I just had a discussion with a coworker after we got it where they told me that.
And yeah, when I was in my younger years when I had more time than money, I might have cobbled together some solution like you are suggesting, but these days, I can pay for the nicely integrated solution that I don't have to debug or worry about.
I hear the Wahoo Kikr is great for cyclists: https://www.wahoofitness.com/devices/bike-trainers/kickr/buy
In practice, it took me a few workouts to translate the resistant callouts from the Peloton to the Keiser. Also, some instructors are better than others about repeatedly calling out cadence and resistance. You can get lost in the workout if the instructor doesn't call them out enough.
I understand the value proposition of an internet-connected "virtual class" for motivation, competitive pacing, social gaming, etc but my limited research showed that one can get these benefits with just the Peloton app[1] (the software) without buying the hardware (Peloton branded bike/treadmill).
The big disadvantage with the whole Peloton hardware+software integrated package is them changing the Terms of Service on the customer at any time to raise prices. (E.g. this thread's topic of abruptly removing treadmill's "Just Run" $0 cost option.)
Is the DIY option with your own bike/treadmill with an iPad running the Peloton app too awkward that it makes the integrated Peloton the better value? Or are buyers simply not aware of the options?
[1] https://apps.apple.com/app/apple-store/id792750948