> the problem is when someone relies on you

I'm not sure what you're getting at. The problem is only magnified with the web: there's only one seed. If a site is down, it's down, and you can only hope Google cache or the web archive got a copy.

That problem is never worse with P2P protocols.

That is only valid when all nodes have an equal chance of being down. Big websites have entire teams dedicated to keeping their site online, so even though it's centralized, youtube is actually a very stable website. A torrent with one seeding peer (or an IPFS object where only one node has it pinned) can potentially disappear if a single machine has a disk failure.

The basic assumption is independence, so I'm not even considering centralised hosting by Amazon or Alphabet.

I am also assuming that if someone cares at all about seeding something, they will make sure it's not down too often.