Low-impact cross-training is genuinely useful for runners with knee issues, and cycling does that job well. The real question is whether you need Peloton's ecosystem or just a bike that doesn't hurt your wallet. A standard stationary bike at your gym handles the physical part just fine - your knees don't care about the brand name or the fancy screen.
Where Peloton actually differs is the coaching, community aspect, and structured classes that keep people motivated. If you're the type who thrives on instructor energy and tracking progress through their app, that's worth something. But if you're mainly looking for a weather-proof way to stay fit without impact, you're paying a significant premium for motivation features that a cheaper bike (or even your gym's existing equipment) can mostly provide. You could grab a basic stationary bike for a fraction of the cost and use YouTube or other free resources for guidance.
The knees thing though - that's solid reasoning for adding cycling. Just make sure you're not overloading them by ramping up intensity too fast on the bike. Start easy, let them adjust to a new movement pattern, and listen to what they're telling you.