I disagree with your conclusion for various reasons, but one of which is that vote sellers inevitably do poorly on curation, even before accounting for (possibly/likely/maybe) higher usage of downvotes. Yes there will get some more SP but it won't make up entirely for the lost liquid payment. Vote selling, on net, becomes less profitable and less attractive.

You made things worse. You just made the 200$ trending post that was payed for cost something like 130 SBD instead of 180 SBD now

I don't see this as worse, or even meaningfully any different. From the buyer's perspective, if you pay 130 SBD and get 130 SBD +/- back or pay 180 SBD and get 180 SBD +/- back, that's all really the same thing. In fact in both cases you have paid nothing, and this, by itself, really won't make any difference to who buys what votes on trending or elsewhere.

The difference is from the sellers perspective, where the seller gets 50 SBD less and has to make it up in higher SP. They won't.