If you work from home and otherwise spend 20 seconds walking for your "commute", a factor of 60 improvement is a big change.

I can tell you that amount of walking makes zero difference to me. Certainly not in terms of weight. Maybe it has some health benefit that statisticians could eek out by studying 1,000 of me over a lifetime, but not anything cosmetic.

And cosmetics are what really matters in these discussions, because that is how you are ultimately judged and categorized.

Maintaining 1lb of fat is ~2-4calories per day though this increases with activity levels. So, assuming all else is the same over 30 years you would end up around 20-30 pounds heaver.

In the real world exercise is important for regulating appetite which has a much larger impact. Remember, evolution was not optimizing for people to sit around all day long. So, feedback systems are dependent on activity.

Exercise won't cause you to lose weight. It has other important benefits though.

It's not a lot, but 20 minutes daily walking at a good pace would be sufficient for most people to lose about 1lb / month, assuming no change in diet.

Like a hundred. Pretty much a large Apple.

A decent paced 5k burns like 400 at most

Most people aren't overeating by thousands of calories a day, increasing activity by 20 minutes without increased intake would probably diminish the vast majority of weight gain for a large percentage of the populous.