All relays are paid relays.
Not everyone will pay for a relay.
Some people will, and that'll be enough.
If only one person out of a million pays for a relay, that'd be...
that'd be...
That'd be a heck of a lot of relays.
Discussion
One per house
One per neighborhood
One per region
Hmm...
Maybe one for planet
Think about it this way:
The architectural necessity for any one person to host a relay is not directly proportional to the number of users. That is why we don't need to put a relay in every client and therefore on every device (this would mean one person might end up with 3+ relays, which is complete overkill).
If the network has 2 users, it'd be best to have at least 2 relays.
The same, however, is probably also true at 10 users and 100 users.
At 1000 users, you might want to add a 3rd relay.
At 10000 users, 10 relays might be good.
And so on.
Seeing it now 👀
What I could be missing then is the ability within a relay to 'sub-relay' in so a manner that a dedicated hardware could give service to different purposes...
As I pointed with the 'house' and the 'region' relay, these already are, —in the sense of the nostr day-to-day experience—, alike theforest's and yabu's respectively: theforest's Stella house where friends and relatives are welcome; yabu's instead assures only people from certain region and language can write there; I haven't seen in that sense a 'neighborhood-relay' one yet... But then I could guess there would be stable fixed relays by the numbers you mention along several ones that comes and goes as they see fit 🤔