While I think it would make more sense for the number of votes to be less than 20 to prevent a single account from dominating the top 20 with their votes I think it doesn't really address the long term issues because whoever is behind that big account can theoretically just split their stake to multiple accounts that they control to achieve the same effect if they want to.
It seems to me that one problem may be related to the "upvote only" nature of the witness voting system (and the SPS voting system) -- the only way to "vote out" a witness whose policies you don't like is to "vote in" someone you like better. I haven't thought through all the implications, but I wonder if adding "witness downvotes" (and maybe the ability to vote with percentages rather than just binary) would help -- then Steemit could decide to not upvote their own witnesses but could protect themselves by downvoting witnesses that run code that they have concerns about, which would be a de facto veto power for them given the large size of their stake.
It might also be a good idea to limit the amount of SP that a witness vote can be worth, e.g. no matter how big the stake behind it the vote only counts for a maximum of x% of STEEM in circulation. (Although this basically has the same problem that I mentioned at the top, which is that a large account can theoretically circumvent it by splitting up their stake across multiple accounts.)
I agree with you that implementing downvote for witness voting could enhance the system. But for now, I would like to start the basic step first.