Firstly, I notice many of you are putting forth the "nobody forgets things like riding a bike, swimming, reading, etc. over a matter of weeks" argument.
To counter this, I would say that most of the skills in the game are not these basic tasks. Learning an incantation, understanding the inner workings and properties of alchemical ingredients, knowing exactly how to forge the ringlets and intertwine them to repair a damaged coat of mail... these are more advanced than basic skills like riding a bike or swimming. Also fighting effectively is a bit more complicated than "swinging a sword". That's why they took athletics and acrobatics out. If you learn the complicated process of curing leather work once or armour once or twice and then don't do it again for a month, there is a high chance you won't be able to reproduce your work as well as you did when it was fresh in your mind. Yes, you would be able to relearn what you lost, hence my whole threshold argument, which most of you seem to skim over.
Secondly, many of you also are against this idea because you think it would be frustrating and annoying, only punishing the player.
Why is it that much of a punishment when the skills that are atrophying
are the ones you never use? Is a burly Nord whose stealth skill was 14 going to see that it has decayed to 12 after a while and be pissed? No, why would he care? And if he does, why would a warrior suddenly want to become a stealth oriented character instantly? Of course any character could transition under this system if they wish, but it would be more of a lengthly transition, which is more realistic. Or is everyone in here whining because they won't be able to easily be the "master of all trades" god? - which is exactly what this system is trying to make difficult. I would call it less of a punishment and more of a keeping the player on their toes.
But yeah, would never be implemented except in the dream of hardcoe mode