Disney clearly has a lot more guests than Universal so short term I don't think there is reason to worry. But, considering how quickly Universal Studios is able to build new rides and "lands", I think that they may catch up to Disney very quickly in terms of size and quality. Disney is becoming very crowded and limiting. It may be good for Disney initially to have a lot of guests and advertise the fact that you get to ride fewer rides in general as a "plus", but the number of guests who are unhappy and feel they are not getting their money's worth will also increase since Disney is not expanding or adding anything to help with the crows.
Considering not all of Disney's guests are die hard fans or loyal (though many are), I could see them spending more time at Universal or all of their time there if it's a more pleasant experience with less crows and less time spent in lines. And of course more rides, more quality. If one compares the experiences and realizes Universal is offering them more for their money, they will favor Universal. It won't happen overnight but there's a possibility that it could happen. If Universal wanted to, they could build a third theme park in the time it will take Disney to build Avatar Land. They seem to be really fast and efficient.
Also, I think that the strategy of building and expanding to attract more people is a better one than attracting more and more people to then very slowly start building more things to accommodate the extra people. It's better for the guest and for the park's image as well. I love Disney and I like Disney's style much better (as in the look and feel of the rides and parks) but I have to admit Universal seems to be doing a much better job in the past few years, basically ever since they announced that they would build the Harry Potter area. Financially, I'm not sure what is best, but I'm of the opinion that quality is more important in this business. Assuming you are making a good profit, you don't need to be the one that makes the most money. Greed is never a good thing.