I think many people are confusing the meaning of “rational”. In the context of philosophy and other related fields, it has a very specific meaning and the thing that you are asking is actually empirical in nature when in the context of those fields. But the belief in those empirical evidence being rational stems from the lack of knowledge on the philosophical side of things so if you really are looking for an answer in good faith, then a discussion is possible as otherwise it is indeed an insanity trying to convince someone that will not accept others definition from a different field.
Yeah, but as you said, it is highly dependent on the implementation. Theoretically it is possible that the user is also seeding the previously downloaded/streamed chunk (via WebRTC for example if using a browser). That reminds me of a madlad that stores data on a ping packet (see suckerpinch channel on youtube, specifically his video titled “Harder Drive”)