I think the mistake here is that you're assuming that the reward pool goes undistributed. My understanding is that if an account holding the standard 14.5 SP placed a single vote, and that was the only vote of the day, it would effectively distribute the entire reward pool to the content that it voted on.

Bidbots reduce your ability to grant rewards & in turn, negatively affect your curation returns. I think if the bidbots didn't exist to delegate to, the overall payouts would be larger as a result of inactive users leaving their stake unused - or perhaps if they were not inactive, the distribution would be wider than just those who want to pay to have their spam upvoted.

I've seen the dilution happen, it's very real - the 'pending rewards' dropdown says 'pending' for a reason.