Yes, i think your are correct but point raised by @xyzashu is wrong or right. I personally think that the point raised in post is very true. The witnesses has more responsibility then a normal steemians, they have to take it. Like their share will also reduce by 20% to 30%, and by SPS witness will be get benefit more because they can more technical then others (in most of the cases). So, they will get major share from SPS pool.