The point is that a "Pixar" themed area could be more diverse, dynamic and flexible with adding or changing attractions than a single IP land. When you do a single IP, land, you are more constrained and dependent on the popularity of that one IP. When you have a land with multiple diverse IPs, then it doesn't matter if one of the attractions has an IP that falls out of favor -- it's easy to not fret about one ride, plus it is easier to re-theme one if necessary. And multiple IP lands are more likely to have wider appeal since there are different things being featured.
A more generic land also allows for much more rapid integration of future new IPs.