Yes, I wrote on this recently too. I showed here what the witness tables would look like immediately if no votes were recast after all of them that were older than both 3 and 6 months were terminated. @reggaemuffin put forward a good idea about having all votes terminated for an account if none of them change during a certain time period. When combined with notifications being sent to the voter, there shouldn't be any huge problems and yet significant benefits.
I agree that reducing the number of votes available to each account is also a good idea, I think that each voter should not have enough to create a consensus, yes, so maybe 15 is a good number.