As of now people are fairly generous and the cost of being on a public relay is probably not very much.  But if every event is sent to every relay, scaling problems arise.

I think of this situation as analogous to posting a blog onto a blogging website, or onto my own website.  I can follow 100 different blogs on 100 different personal blog websites.  Or maybe I can follow them on a few centralized blogging websites like medium, substack, etc.  And it works suprisingly well with zero redundancy (just one website).   So if it works so damn well with zero redundancy, do we really need to be 25x redundant?  Even being just 3x redundant is a huge improvement over a blog on a website.  Being 5x redundant is really a hell of a lot safer.  I can't imagine a scenario where anyone would need more than 7x redundancy (except highly hated people like Alex Jones and Donald Trump).

Except actually I can imagine such a scenario.  If it came to be that popular nostr clients didn't use the outbox model.  Then people would have to post to all the popular relays too.  But even then, I don't see the benefit of going beyond 7x and just picking some of those to be the popular relays, and instead I start to see it as maybe being disrespectful, taking advantage of the goodwill of open relay hosts because you can...  potentially leading to those relays to shut down due to being overloaded.

And many have predicted all relays will eventually charge for service. lest they be taken down for illegal content or be flooded with excessive traffic that the operator cannot afford on their goodwill. It might come to pass.