> There was completely no sense to run a relay except for altruism.

Which has always been a *massive* motivator, often sufficient on its own, in the field of tech, especially software.
Many platforms, organizations and projects are run for this purpose alone.

I do not understand why Nostr specifically needs to have economic incentives, or really any incentive other than furthering the fundamental human right to freedom of speech.

Obviously you need economic incentives for "enterprise" relays, but I disagree with OP anyways.
Enterprises haven't been what has promoted freedom in anything related to software, and I don't think they will be.