@LejonBrames, no disrespect intended here, but there are a lot of assumptions in your question that I ... question.
Swings alone, and the results thereof, will depend a lot on how you perform those swings. It's perfectly possible to swing without truly standing up straight at the top of the swing. I know, I've performed far too many swings in this manner. Another point - swings with light weights won't provide enough challenge to fix much in the way of posture.
I spend most of my time seated - at a computer, at a piano keyboard, or in a chair while giving music lessons. When I get up from my chair, I make a real effort to rediscover good, relaxed, tall posture. Related to this, I walk a lot, and if you were to watch me, you'd notice I'm kind of wriggling around (for lack of a better way to put it) a fair amount during the beginning of most of my walks because I can feel my posture is still a bit off, still a bit "hunched" or whatever words one wants to use here. Thanks also to the blessings of my bad back, it doesn't realign itself easily, so it's something I am more or less forced to pay attention to.
All other things being equal, swinging a kettlebell will be better for one's posture as one ages than not swinging a kettlebell, but what is "enough" is something each person must ultimately, with the aid of good counsel in the form of coaches and teachers, learn for about themselves, for themselves, and accept responsibility for maintaining. That maintenance may be just swings for some people, sure.
-S-