I lthink that s an interesting idea. At first this sounds great, especially after the grand return of Mr Autofellatio, but as many pointed out, it just solves a problem by creating new ones:

• It could make the witnesses vulnerable
• Big stakeholders could approve 20 and disapprove the following 10
• Witness wars

I like better the possibility of a decaying vote. Yes, maybe some users will create bots to automatically vote again for the same witness, but if they do so, it means they remain engaged with them. Fine.
But it solves one major problem : without decay, the witness votes from dead accounts will be here forever. If for any reason, @freedom was unable to log into his account, the top 20 would be written in stone with the current implementation.
Whether the vote should completeley disappear or use a half-life model is another discussion.