"In essence instead of BidBots why not use on-chain marketing to fund something like this?"

I can think of one reason to implement a regressive tax on the ~10% of rewards not currently extracted by substantial stakeholders. Only one, and it isn't to fairly distribute the expense of funding development to those that will benefit from it. Whales currently extract ~90% of rewards, despite creating almost none of the content those rewards are supposed to encourage creating. This tax will be extracting yet more rewards from creators and enabling whales to vote to fund SPS proposals that will provide kickbacks, just like governments do today across the world.

That's the only reason for this particular funding mechanism I can grasp.