It then becomes easier all else being equal for one large stakeholder to elect some witnesses
But we are not talking about "electing some of the witnesses". Sure they could get a few of their people in, but they wouldn't be able to get all 20 in like is possible (and easier than a model with 5 votes) right now.
It isn't really clear it is possible to vote in 20 witnesses now (only steemit's stake is close, and may or may not quite be there, I'm not sure), and IMO the best solution to that would be try to increase the vote weight on the top 20. One way to do that is to increase the vote limit even more.