Knowing that number is essential to prove your claim that Disney looses money by charging for parkhopping. Without it all you have is a hypothetical maybe.I don't know because Disney never shared that information with the survey company. But it's not the first time disney refuses to share info about how many park hoppers they sell. Check this article out.
http://articles.latimes.com/2013/jan/09/business/la-fi-disney-passes-20130109
During my time as a TA about 60% of my clients purchased the parkhopper option and that was without me pushing it. I even recommended against it in some cases. I can not imagine my numbers being higher than what occurs as a whole considering that both Disney and most TAs push that option.
The bottom line is, as you stated, comes down to simple math. If the money they make by selling parkhoppers exceeds the hypothetical amount that they are loosing in lost sales, then selling parkhoppers is the right thing to do. Considering that Disney knows exactly how much they bring in by selling the parkhopping option and has an educated estimate of how much is left on the table by those who do not purchase it, it is fairly safe to say that the simple fact that they continue to sell it confirms that there is more money to be made by selling it.