In the alternate history I'm imagining, with network-layer accounting and payments from the beginning, flooding and spam were never much of a problem -- they were at most a "nice problem to have". Endpoint security became a priority from the beginning, because failures that cost money (even if not much per computer) get treated that way. Participating in the network as a full peer, e.g. running the equivalent of a webserver on your home PC, was considerably more practical and normalized. (The bigger address space of course went into this too.) And this history made other peer-to-peer software easier to develop and more practical and economically sustainable, in a virtuous cycle.

This doesn't mean it'd kill advertising. It wouldn't even kill all of the forces encouraging me to put my 'content' on centralized services even though I'm nothing like a media corp. But I suspect the right changes could've helped a lot towards a healthier computational ecosystem now.

This early architectural decision that costs are out of band didn't fundamentally make anything impossible. But when you see a lower-level problem addressed by higher-level workarounds, and you're getting to redesign the system, isn't that exactly what constitutes an opportunity?