I am quite new here also (only a few months in) but maybe I can try to answer some of your questions and if I'm wrong I hope someone more experienced can step in and correct me. #nostrnoob

You struggle to understand why someone would run their own relay but same time question where media and posts are hosted and what if the node/relay goes offline... well, that is the incentive for running your own node, piece of mind and personal security/stability in the network. One example for simplicity is to run an umbrel node, costs a few hundred dollars and comes with a Linux based operating system that allows you to install apps to run BTC node, LN node, NOSTR relay and personal cloud server (and more) all in one device, though I think an external NAS/drive system would be required to make the cloud server fully functional 🤔 you could also build your own device and install the OS.

As to monetisation, there are already premium nodes you can subscribe to, they tend to improve the speed of your feed when loading and perhaps some of them might provide some curation of content? Or creatives might hide some of their content behind a pay wall in this way (I think)

You could argue that the cost for using NOSTR in the long run is not so much in subscription fees (although as mentioned it is kind of possible with the premium nodes) but running your own relay is the cost of doing it "properly" (i'm not there yet, but I hope to be), or you can just trust that there will always be enthusiasts out there willing to pay for others to use the protocol I.e. #noderunners, just be sure to tip with a fat #zap if you ever come across a dev or node runner, they do us all a great service out of pocket!

I hope I can be corrected on any inaccuracies here... #asknostr