Actually it's the other way around, that's why:
I would rather go for increasing 30 than decreasing it.
Problem is not with those who has high stake that are voting, but with all those who has their stake and are not voting.
When you take a look at number of people voting: highest voted (number of votes) witness (not me) has ONLY 16k votes. Top10 has less than 9k, Top20 less than 7k, and Top30 barely 5k.
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.