SBD is supposed to be a stable token to facilitate commerce. That may not be prioritized by Steemit Inc any more, but it's still worth pursuing.

And how do you know those cashing out SBD are buying STEEM?

I don't, but whether they are or not, every stakeholder has the power to direct this excess purchasing power towards removing liquid Steem from the market. This would be the SBD peg in action. While the SBD peg is broken, it makes more sense to do this and to raise the price of Steem via constrained supply, than to try and plug the leak of selfish voting.

Once the peg is fixed those who purchased SP this way will have more power to police when the money is coming primarily from the community again instead of SBD speculators.

I am usually one of the ones who downvotes self voters. You can look through my history, I have regularly burned my voting power down to near 0% on downvoting self voters alone. However right now trying to police self voters is a bit like enforcing a ration during a bounty.