My kidsd are spaced out far enough that even after one outgrew the stroller (around age 5), I still had one little enough to be in a stroller, so we still brought it. And sometimes used it for the older child.
Stamina-wise, my older two (now 10 and 6) walked all over
Disneyland, California Adventure, and
Legoland with no problems. And the times when just the big kids and I went back to the parks with no stroller at all were so much fun. Hop on the train, hop in line, never have to worry about parking it and fetching it.
Having said that, the last time we were at WDW my oldest was 7, and we had a rented stroller, and we used it. When the parks are jam-packed, or we need to get from one place to another quickly, I felt she was safer in the stroller than dragged along behind us in the crowd.
Outside of theme parks, my kids haven't used a stroller since they were 3 or 4. My youngest will be 3 in November, and she still uses the stroller, although most of the time she gets in it under very loud protest.
The obesity epidemic is caused by lots of things; probably excessive stroller use is part, but TV watching, video game playing, parents concerns over letting their child play outside, and increased reliance on junk and convenience food probably play much bigger parts.