I really think that when there's a choice to be made between drastically upping one's exercise and gaining small amounts of weight, it becomes a quality of life issue and only each person can make that decision for themselves.
Hypothetical. Let's say I'm a woman going through menopause and I weigh 130. I (hypothetical imaginary I) exercise an hour a day. Now, suddenly, to maintain a weight of 130, I have to exercise five hours a day instead of one. If I don't do that, I go up to 140. So, do I give up four hours of whatever hobbies I enjoy, or do I just say to myself, "You know what? I'm 140 now and I'm OK with that."
TurtleDove, I think some of the disconnect here is that you are a person for whom exercise is your hobby. It's your passion. It gives you more joy that anything else on earth. And that's great! We all need something to be passionate about. But for a lot of us, exercise is a grueling chore. There's an amount of it we're willing to do (just like any other chore, like the dishes), but we don't want to devote all our free time to it. You would almost certainly welcome four more hours of exercise in your day. To get an idea of what we're trying to say, imagine that someone comes up to you tomorrow and says, "By the way, TurtleDove, in order to keep looking exactly like you look today, you need to give up four hours of exercise every day, and spend those hours cleaning tile grout instead." (Substitute something else if you actually love cleaning tile grout! LOL!)
People are not all going to make the same choice when faced with the options of "gain a little weight" vs. "give up all of your free time." Because we value different things and have different priorities. And if someone has a little pang of "I like the decision I made, but I do sometimes miss my college bod," it doesn't mean we're uncomfortable with our decision on a deep, existential level. It's just human nature to be nostalgic sometimes, and to wonder about what other choices might have been like, in any area of life.