Meh. If witness votes expire frequently then all large stakeholders will just automate renewing them. I know I would.

I'd be in favor of dropping votes from fully-inactive account though. It is a reasonable debate as to whether that means six months, 1 year, 2 years, etc. This wouldn't so much be about campaigning though, since those fully-inactive accounts are likely people who are gone. It would be about not having stale/absent stakeholders still exerting influence, and yes that would include some existing witnesses losing support, which they would have to gain from other voters, or permanently drop out.