If votes decayed, Steemit would become a playground for non-stop vote begging and campaigning. It will increase aggressive behavior between witnesses similar to government elections. WItnesses will focus less on doing good for the blockchain and community and focus on maintaining their position.
Will also cause trusted witnesses to lose rank if people forget to renew them, eliminating the power of your vote.
It is tough enough to get users to vote for witnesses, this would increase the difficulty and amount of work users have to do.
Some will counter this by creating automation to ensure their votes are enforced.
I don't think it will work any better than what we have now and will, in fact, make things worse.