I kind of agree with you, but I don't think it's immediately obvious.
Is there an intrinsic, rather than pragmatic, reason why mass obesity would be a bad thing? To use the given WALL-E example, what would be so bad about living in that world?
For such a magic pill to exist would be wonderful and I'd love to have that option, but WALL-E as an example undermines your argument...
We already see those 2 problems right now: Existing issues about how to handle people who don't fit in economy seat on a plane without disturbing the next person. Smartphone (and previously calculator) design doesn't work for people with bloby fingers.
We barely started to make essential services available to disabled people who don't actually have a choice of moving on their own, or read, or... If we can't handle that special case, I don't think we can easily handle those self-inflicted issues as a norm.
I feel like we'd become better at accommodating disabled people if there were more of them. And that would be good for people who don't have a choice about their disability.
As an example, "large screens for fat fingers" also sounds good for people with poor motor control.
It's not like we don't accommodate disabled people because we hate them. It's because it needs serious redesign everywhere. If every other person became voluntarily disabled today's cities would become unusable. It would likely have a social split between people moving out to suburbs and flat towns and active people who can use existing dense infrastructure. It would have high impact on taxation to support the change. It would have ripple effect on everything.
Now, would this be a net positive change just because we also improved life for those naturally disabled?
It's not for me, personally, but I'd love if we had more blobs of fat! I'd maybe even start a fat blob washing company!