The appview needs to index the relay contents to build a view of the whole network, the relay is just a type of CDN, and is NOT that expensive. There are multiple individuals maintaining full copies right now, while the network is at double digit millions of users. The relay is by far less expensive.
You're looking at poorly matched cloud options in that article. The individuals doing it does it easily on a NAS and equivalent. The cost from running public relays will come from traffic, not storage
It's the appview that needs to be made lighter, and that work is progressing. Like building variants you can self host which are selective and only care about content from your network (and only fetching other content as needed)