IPFS hashes are permanent. As long as there is a node somewhere in the world hosting the content, it is there.
You could setup a node that hosts your own content, and this way even if everyone else deletes it, its still there :)

Having users host their own content is sub-optimal, and so an incentive structure will be created, whereas people hosting the content get paid.

The STEEM blockchain hosts the post itself, rewards and the IPFS multihash.