That's an awesome thoughts and pulled a hardwork to deliver with statistics. I believe yes , decreasing the witness votes will be great idea. I agree with that decrease the large stake influence

But it's also effects the low ranked witnesses who are running the block chain with minimum profit or returns. The decreasing of witness votes will discourage or leave them hopeless.

But there is another great way to counter this problem.

Expiry of witness vote

Yes, after every period of time, the votes of a member should be expire and again he need to vote. So the witnesses are in constant check and attentive about the votes. If the top 20 witness is inactive on chain, people don't vote for him again. So there will be regular events by witnesess showing their work to block chain members and urging and reminding about the importance of the witness vote.

Most people even don't know about this and most people opts proxy instead of checking what actual things going on.