A Peloton could definitely help with low-impact cross-training on rough weather days, but whether it's worth the premium price depends on what you actually need. If your main goal is just having a stationary bike to use when running isn't feasible, a regular gym bike or even a cheaper stationary option does the job just fine. The low-impact aspect is real - cycling won't stress your knees the way running does - so that part of your thinking is solid. Where Peloton differentiates itself is the community, the structured classes, and the gamification element that keeps people motivated. Some folks swear by the coaching and the variety keeping them consistent; others find they could've gotten the same fitness benefits from any bike.
The honest answer is that the bike itself doesn't magically do anything different than another decent stationary bike. You're mainly paying for the platform, the instructor library, and the competitive leaderboard stuff. If you're someone who gets bored easily and needs that external motivation to actually show up, it might be worth it to you. If you're already disciplined about your running routine and just need a backup option, you're probably fine grabbing a basic stationary bike or using what's already at your gym. One thing worth testing first: make sure cycling actually feels good on your knees before committing to any bike. Sometimes what helps one person aggravates another, so maybe do a few sessions on a regular gym bike and see how your knees respond before dropping that kind of money.