There is a problem with this approach I believe. If all votes expire concomitantly, how does the blockchain reach consensus, even for a short period of time? And there is the possibility of huge anomalies immediately after, before some sort of votes coagulates.
If there is a period of grace to form 'new government', while the previous top is still used, it makes sense, but not sure if it doesn't introduce complications into the blockchain code (true, less than a set expiration time would, probably).
Update: Overall, when all objections and limit-cases have been considered, I believe this is worth testing.
Update 2: Here's another potentially serious drawback: HF will become undesirable by top witnesses, because there is a chance they lose their positions. They might vote against a HF not because it's bad for the blockchain, but because of pure self interest of the witness.
A possible solution would not to have it on hard forks but on a rolling 3 year per user account so no witness would lose all there votes unless they somehow gained all the votes in 1 day. This is plenty time to campaign for votes without constantly spamming everyone to fork melting fury.
Yes, I think it makes sense. Even 2 years or 1 year would be fine. And doing it on hard forks relieves the blockchain code from constantly verifying the vote time of every account for every witness, which would be the case for a regular vote expiration check up.