Defining the best number of witnesses you can vote for (with full stake) is not an easy task. I invite you to read this interesting analysis from @raycoms.
One voting scheme he did not include in his analysis is "infinite witness votes with stake spread", i.e. you can vote for as many witnesses as you want but your voting power is spread among them. It would be interesting to see how it impact the current witness ranking.
Requiring votes refresh is cumbersome and would lead to witnesses permanently campaigning.
On the other hand, removing/decaying votes from inactive accounts is an idea I already talked about.
RE: Is Hive decentralized and who governs it? Episode 2