16k voters can hold super majority of SP behind them though, I believe the plankton feel like they have no effect on top witness positions, I got close to 6k SP and even I feel like that.
Limiting votes to 7 or so would get people a lot more involved because they can really have an effect, this would ensure more competition for these witness spots and more value would be gotten from these witnesses that are able to get these positions since they would have to fight for their positions.
But those are my thoughts at this moment and could be wrong.