I think you have missed the point that CryptoPrometheus was trying to make.
Voting decay means the overall level of votes will go down, due to voter apathy. That means a large BTS holder can vote in not only one witness for himself, but as many as he likes, taking over control of the blockchain.
Note that with 27 active witnesses, buying out 51% of them witnesses will cost at least 14 times as much as buying out one, whereas you currently need less than twice as many BTS to vote 14 in than you need to vote one in.
I get that voter apathy may be a concern in the future, but at the moment the low level of voting participation in the first place is more of a concern than voter apathy IMO.
At the moment, when you're voted in & your node is running you're very unlikely to be at risk of losing your elected witness position (a potential centralization risk). I don't think that users will become apathetic to voting if their vote degrades over say a 6 month period.
We should be continuously driving users to vote (and re-evaluate their votes after a certain period of time) within the Bitshares network. We should be brainstorming new ideas regarding how we can drive up voter participation in the future, such as holding informal fun polls (favourite colour, community mvp polls, etc).
The Gridcoin network is also experiencing a lack of voting participation - only 1/7th of total vote weight votes which brings the validity of poll results mandate into question.