a dynamic site can't work without mutability, ipfs can't deal with mutability, so ipfs comes with ipns, which allows you to statically reference content that might change.
Yeah, IPFS + IPNS is effectively on par with Dat archive mutability, but mutability being built-in to Dat + its verifiable history log is particularly well-suited to building peer-to-peer websites
Of course, the design of IPNS makes it impossible to prove that you've got the latest version of a name's value, and makes it relatively easy to attack. I don't know if Dat has the same issue, I haven't looked at it.
It's not impossible by design, it's simply a feature that hasn't been implemented so far. IPFS is by design pluggable on all layers and thus theoretically capable of a ton of stuff.
So IPFS is, according to you, not defined as its protocol, but instead as its API? So I could build an IPFS implementation that just grabs stuff from my (centralised) web server and say I'm using IPFS? Either nonsense, or useless - the goal here, I thought, was to build a global decentralised filesystem that looks the same from everyone's perspective.
All DHTs suffer from this issue. It's just particularly likely that IPNS's use of a DHT will lead to attacks. Making it not susceptible to this would require a redesign of IPNS's protocol.