Would IPFS or DAT not suffice there? How is 'another permanent storage, that is a piece of a larger project, better than one that has the sole purpose and focus?
The challenge with IPFS and DAT is that you have no guarantees around the data reliability. The DHT style of p2p sharing pretty much only works for popular content. Incentivized storage networks can onboard any type of data and guarantee high uptime.
It's also been my experience that IPFS has significant performance issues. If you use a professional gateway like ipfs.io or cloudflare it runs at good speeds but as soon as you switch to being fully peer-to-peer it's almost unusable.
I don't have much experience with DAT, it may not have the same performance issues.
disclaimer: I work on an incentivized storage network called Skynet
Those are but some of the problems that IPFS, Dat (or Storj and probably Skynet too) do, or will face.
That doesn't make "rolling your own" any more reliable, performant, etc. So the question becomes even more apt: why will "building your own decentralised storage as requirement of a much larger project" work. If even the focused, dedicated "decentralised storage projects" cannot solve some problems?
It's also been my experience that IPFS has significant performance issues. If you use a professional gateway like ipfs.io or cloudflare it runs at good speeds but as soon as you switch to being fully peer-to-peer it's almost unusable.
I don't have much experience with DAT, it may not have the same performance issues.
disclaimer: I work on an incentivized storage network called Skynet