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. 

 'I can't imagine a scenario where anyone would need more than 7x 
redundancy (except highly hated people like Alex Jones and Donald 
Trump).'

Well given that Trump  is running for president (no matter how much anyone hates him) I think this makes for a very strong argument against Nostr being hugly censorship resistant.  It would be easy for 7 relays to all agree on using the same censorship lists.   In fact I would say the ability for Donald Trump to never get censored on Nostr on most peoples clients would be the bare minimal needed to claim Nostr is actually censorship resistant . 

 The biggest problem with paid for relays for non bitcoiners isnt the cost so much as  having to disclose yourself for payment details (reduced anonymity) , inconvenience (its boring and complicated buying bitcoin).  And cost is still a factor for some when there are free alternatives , but for everyone when they aren't sure if they will even like Nostr.  Subscriptions are a faf.  There are so many,  getting increasingly frustrating and difficult to manage. It doesn't feel like you won your data when you have to pay.  That doesn't make Nostr sound as appealing as it once was.  One of the big fears against paying for Twitter is that it removed anonymity.  I suspect a lot of people don't pay more because of that than cost.  How do we verify relays aren't spying on us and linking our payment details to our posts?   I think being asked to pay for nostr will kill it.  Its a major obstacle for so many reasons, but even when it is free right now people are reluctant to switch to nostr.