I mean, let's be real here. IOTA is a blockchain in all but name, IPFS is substantially blockchain-adjacent, "p2p" is vague to the point of meaninglessness (and isn't actually a registered method), and "web" is silly (a web site is already identified perfectly well by its URL).
My point is that P2P isn't a single thing -- it's a whole multitude of things, many of which don't make sense to create a DID for. Gnutella is a perfect example of where it wouldn't make sense: the Gnutella protocol didn't provide any way to create a persistent reference to a file that was being shared, and it'd make even less sense to tie an identity to such a file.
It's a bit like comparing monkeys and apes though. Yes they're all primates but the family tree does matter and you can't just mash them together.
IPFS resembles many previous attempts at distributed file storage, which did not use blockchain. They had other ways to encourage fairness, which appears to be the primary use of blockchain in IPFS. The existence of the concept of Merkle trees, named or unnamed, lead to blockchain, not the other way around. And it has other children, like some digital signature specs.
