Whilst not exactly focusing on vote decay, this piece of work i did recently may indicate just how little 'inactivity' affects witness standings.

https://steemit.com/witness/@abh12345/do-inactive-account-votes-for-witnesses-affect-the-witness-ranks-1540900963927

I am also for vote decay / recasting votes over time, but I think this is something that will show more of an effect in years and not months.

Cheers.