You're refusing to pay for a product today based on what they might do in the future? Does the future possibility degrade today's product? Can't you just cancel your subscription if ads start showing up?
In some cases, yes. I specifically don't use Spotify in part because I can possibly lose access to my playlists and music I listen to in the future. Building a music collection is a big part of listening to music for me, so the future possibilities of the service actually degrade todays product.
I don't watch streams so I don't have any input on Twitch, but I could imagine someone having a similar outlook.
I don't think you lose access to any of this when you unsubscribe, I think you just start seeing ads again when watching the live stream.