Like I said, watch some of the videos on that channel. It's pretty obvious. And the amount of people overweight in the video I posted is rather small compared to today. If you count in that video I shared, there are roughly 100 people seen total. I saw 0 morbidly obese people, 2 obese people, and only 14 people who are overweight. And only 1 of those 16 overweight/obese people was a child. Way lower than what you would see now. Just look at a photo of crowds in 1990-1995 compared to a photo today and do a count. The obesity rate in America has grown nearly 5x what it was only 30 years ago. And now 63% are overweight. Only 39% were overweight in 1997. I would guess in 1990 it was probably only 35%. The numbers don't lie. There are more overweight and obese people today than there were 20+ years ago. Yes there were obese people in 1990, but it was few and far between (as I stated that number has grown 5x since '86). But it is an established epidemic that has sadly grown way out of control now in America.
But I digress, people do rely on the strollers way too much now. As you say, people either are underestimating their kid's own stamina or think the kids need to be wheeled around more than they actually do. The issue there is that contributes to the other issue of people being overweight.
The biggest reasons though are higher population, more crowds, bigger strollers, parents using them more than they should or when they don't have to.
The point about people taking their families at younger ages may also be very true and is a great one, too. Although my family took my siblings and I for a day at DL by the time we were 1, we didn't go to DW on a full blown Disney vacation until we were all over age 8. And like somone mentioned, the cost of a trip is so great now, people want to utilize those children under 3 tickets as much as they can. All of these factors have contributed to the over abundance of strollers.