I don't think this would be good for the public net nor would I support this attempt on the public net, but I would absolutely support a sister chain attempting to implement absolute zero block producer rewards for top 21.
What if being selected with the responsibility of producing blocks was reward enough? What if the reward for producing blocks was akin to the honor of selecting who produces those blocks?
This would spin incentives to sell votes out the window without barricading free-market principles. You can try to sell your votes... but if you get voted into the top 21? You've assumed the responsibility of maintaining the integral network infrastructure.
There's no mix of financial incentives within producers, as the process of gaining votes necessary to rise near the top 21 would entail gradual fundraising to acquire the bare metal equipment necessary to assume the honor of producing blocks.
Edit: Having top 21 BPs earn nothing also incentives rotations of top 21 BPs as those directly below them is raising the money needed to gain the honor of producing blocks.
One of the major blunders that plagues crypto startups is treasury management. This would encourage BPs to make transparent strategic planning where a BP in #22 would be able to save enough to propose to the free market that they can, as they likely have before, produce blocks for expected durations before preparing to rotate back out of the top 21.