One of the key bets behind the design of Indra is the idea of being able to meter all traffic, relays don't forward any packet except one that contained a session key pair. This kills the marginal value of spam instantly and linearly according to data volume (not just number of messages even).

There just isn't such thing as free traffic with this design.

You could imagine a situation where you get paid, directly, to read/view advertisements. Advertisers would be a lot more careful about who they send marketing to if they had to pay for it.

Every system with freebies ends up a rats nest of free riders.

Nostr eliminates much of the attack vector from aggregators, or at least opens that up to be user controlled, but the spam problem is going to be big once enough users are on the network.

Hopefully we'll have you covered long before then.