Well, I wouldn't say "everyone" goes to IOA -- there's still a devoted customer base for Disney, especially with DME and internal transport making it easy to just stay on the resort. Plus, a lot of people are among the toddler set and there's a perception that Universal doesn't have as much to offer young kids.
That said, I agree with the point that DHS is the lower lying fruit and the one most likely to lose days to Universal. It's similar in style and theme to the Universal parks and, while it has some great attractions, arguable has the least to do. Especially for repeat guests who have already seen the shows. DHS is very vulnerable. If something isn't added to the park quickly, we could certainly see attendance drop relatively quickly there.