The way IPNS uses the DHT faces some deep challenges if you really need the “latest” IPNS update (if you don’t there are various ways to make IPNS perform significantly better, see https://discuss.ipfs.io/t/ipns-resolution-takes-a-very-long-... for guidance).
There is also some work to make IPNS work over PubSub independently from the DHT. That work is being tracked at https://github.com/ipfs/go-ipfs/issues/6447, and should significantly improve IPNS performance as well as add in features like allowing users other than the author to keep IPNS records alive (so no 1 day expiration issues).
As for the decentralized nature of pubsub it is essentially an opt-in system. Random people on the network are not holding or forwarding your messages as they would in the DHT. However, anyone who has subscribed to a topic will propagate messages for you to other subscribers. This means subscribers do not need to directly contact the original IPNS record publisher to get a record, but instead can get it from anyone who has it and is advertising that they do so.
it's possible that some miners choose to store their data on S3 or some other large/well known cloud provider. However, miners have other options as well. For instance, they could store their data in their own data centers or with more start-up storage providers without needing all the marketing resources that Amazon, Microsoft, Google, etc. have.
One of the use cases I'm looking forward to seeing from Filecoin though is for regular folks with desktops that have extra storage capacity to just rent that capacity out. Right now the extra 500GB of storage space I have is just an unutilized resource waiting to be made use of. Additionally, it allows people to basically swap data capacity so I'll store 500GB of data for other people, and in return I'll use that Filecoin to pay other people to store my data. This is could be very useful for personal data backup where my primary requirement is that my data live somewhere else that is not on my computer.
There is also some work to make IPNS work over PubSub independently from the DHT. That work is being tracked at https://github.com/ipfs/go-ipfs/issues/6447, and should significantly improve IPNS performance as well as add in features like allowing users other than the author to keep IPNS records alive (so no 1 day expiration issues).
As for the decentralized nature of pubsub it is essentially an opt-in system. Random people on the network are not holding or forwarding your messages as they would in the DHT. However, anyone who has subscribed to a topic will propagate messages for you to other subscribers. This means subscribers do not need to directly contact the original IPNS record publisher to get a record, but instead can get it from anyone who has it and is advertising that they do so.