Jeff wrote:
Animated films cannot have gaps like this. Their audience ages out.

I think for the bigger franchises, such as Toy Story, the audience retains interest and a new generation of kids jumps aboard.
But I agree, it's kind of a crazy amount of time in between. My son Jonah was 6 years old when part 2 came out and he'll be 10 when the next one hits. Might not be avoidable though, these movies are incredibly expensive and time-consuming to produce, even with computers.