Hadn't thought about it that way, but you're absolutely right. So long as spam isn't an issue, there's no demand for paid relays. No one feels any pain from not having them. Therefore, the best way to drive demand for paid relays is to generate spam.
That said, I wouldn't accuse any particular paid relay operator of being behind the recent spam attack without some pretty damning evidence, and I don't think anyone has accused any particular relay operator. At least not that I have seen.
Chances are that the bulk of what most of us would consider spam is just being perpetrated by people who like to cause trouble simply because they can, or because they think they are making a point.
Spam on public relays is inevitable, no matter who may be behind it. You will find it on any sufficiently large network where the following are true: 1. It's free and easy to create new identities on the network. 2. New identities can write data others will see without a cost.
On Nostr, we're not able to make it difficult to create a new identity. Nor would we want to. That means our only option for mitigating spam is to impose some form of cost for new identities to write data to relays that others will see. Public relays impose no cost at all, so they are guaranteed to have spam.
The three typical means of imposing costs to writing data to a relay are social, computational, or monetary. WoT relays would be an example of social cost to gain write access. PoW relays would be computational cost, but we hardly see any of these in the wild since most clients don't support adding PoW to notes. And, of course, paid relays come in as the monetary cost option.
It's a tough nut to crack, though, when you are trying to encourage the network to grow. We WANT new users to come here, and we want them to have a pleasant experience so they will stay. Adding social, computational, or monetary costs in their way is antithetical to that goal. Yet, new users are also the ones most likely to be spam bots that will make the experience of new and existing users alike unbearable. Costs should therefore be imposed in such a way that they are virtually unnoticeable to legit new users, but are prohibitive to spammers.
I have a couple ideas how that might be achieved.