Just because it may be mathematically impossible if EVERYONE tries to do it, does not mean that some won't attempt to - even feel further incentivised to do so. It is the sort of situation where perception is reality and this comes down to understanding human psychology. It is game theory, not mathematics.

I know for a fact there are people who haven't used vote-selling services in the past but are now considering doing so to "stay ahead of the curve" (pun intended).

Some sort of consensus amongst whales, witnesses and vote-sellers about incentivising more vote-selling not being a "socially desirable" outcome would be good to see. I won't hold my breath ;)