It's not sound logic for more reasons than you state. Moving fat people isn't the stated goal of mass transit. It's also not the responsibility of mass transit to give people a workout. Fat is a public health issue, not a public transit issue.

We need to consider this outside the scope of fat. Moving sidewalks work inside airports. I want to see if they can work outdoors. And then, I envision a pedestrian-only downtown area with city blocks where each edge of each block is 90% (by distance, not area) moving sidewalk. This would get people to their destination within the pedestrian area faster. When they get there, they don't need to find a parking space. Without a need for parking spaces, we have the room for moving sidewalks. Personal vehicles can be parked at a parking garage on the edge of this larger ped-only area. Freight and maintenance vehicles can use what road remains. The desired result would be to reduce the need for cars in an area, and expanding that area, not blindly, but smartly so people aren't simply forced to be without a car, but instead have a real alternative.

Now, maybe that wouldn't work out so well. Maybe it would. Ped-only areas are here and there, mostly small, mostly catering to small expensive local restaurants etc. I guess my point is I don't see why I should throw these dreams away because people are fat. And intellectual thought stops there. Sounds like giving up on a problem by avoiding it.

You already know one philosophy. You don't "unlearn" it by learning another. You just learn more. Gain experience!