Honestly, I have no idea where IPFS's "permanent web" thing comes from. Content on IPFS is only available as long as people seed it (or "pin" it, in IPFS jargon). If there's only one ipfs user with a certain piece of content left, and he removes it, it's still gone forever! I don't know why they seem to be acting like ipfs content is any more permanent than torrents are. Regardless, I still believe it's a great step forward, and they have my support.
I think that it would be very durable. the reason is that the Internet archive (or failing them, the Archive Team) would make the effort to pin every IPFS page. even if keeping them live is cost prohibitive, parking them on an offline storage cluster until the cost comes down is likely an option