There were also few people in wheelchairs, etc because back then, they just didn't go anywhere, just as you see very few minorities in old photos. A family did not bring a baby or preschool child out in public much, and families rarely went out to eat. Of course, most mothers still didn't work fulltime, and a plane ticket could cost close to a month's wages for blue collar workers, so most vacations were within driving distance. A WDW vacation was out of reach for most families, unless they lived close by. It was truly a dream, once in a lifetime trip, back then.
Times change, and so do family situations. Some changes are detrimental, while others are a vast improvement, but we can't go backwards.
My children used strollers in WDW (but not at home) until they were in Kindergarten. After that, we just took things at a slower pace. But because they HAD been to WDW in a stroller when younger, we didn't have a need to see everything, anyways.
If children are getting plenty of exercise, eat healthy, well balanced meals, and sleep regular hours at home, I have no problem with them using strollers, eating burgers and fries, and staying up late at WDW...as long as it is the exception, not the rule.
Riding around WDW in a stroller is not going to make a child overweight; he/she was that way before the trip. The increased use of strollers, ECVs, etc at WDW is not the cause of obesity, but the result of a bigger problem. And a certain percentage of the increase (I don't know the exact number, and choose not to make up my own) is also due to an increased number of people who were unable to come to WDW in the past because of physical limitations but have been given the means to do so now.
So yes, the guests at WDW now are most certainly different than those who visited 40 years ago, but so is our society.