That is a very valid point. I think it comes back to the question for the experts about the transmission risk in a busy theme park. Honestly, what Disney should do is request and evaluation from the CDS/NIH/Task Force and then base the decision on their determination.
Without any expert opinion, I would personally not want elderly people visiting WDW right now. They made that recommendation for cruise ships, but cruise ships are certainly MUCH higher risk for spreading illness than a theme park. Every time there is norovirus on a cruise ship a large percentage of passengers get infected. From memory, I can't recall any illness that had an outbreak at a theme park. But, for the elderly and/or people with serious underlying medical conditions, I would think that they should avoid theme parks out of an abundance of caution.