If you learn to make something, you only going to get better at it and will under no natural circumstances degrade in said ability over the course of time that this game is about. I would be fine will producing an item with less quality or having a higher chance of breaking the components if you haven't done it for a month or more, but suddenly not knowing how to do something you could a few days/weeks ago is silly at its core. I understand the gameplay balance reasons for wanting some sort of skillrust or skillcap but surely there's a more reasonable alternative. Then again it might need to be that area where sacrifices for the good of the gameplay must be made, maybe the radiation-or-what-have-you is messing with everyones memory.
Ah, I forgot to include another point in my earlier post. Another planned thing is to split skill levels into an "applied" and an "actual" level. Skill rust rusts away at your applied level, which is the one you use for all of your rolls. However it can't wear away your actual level, and that is used for tracking what is rusted and what isn't. In that method you will never "forget" how to do something, though your ability to actually perform the task that you remember how to do could certainly do so.
As for the increased speed of decay; it's always going to be there somewhat. This is to counteract the fact that in the real world learning to do many of the skills the player learns in hours would take months-years. In the real world people have to train for years to reach the higher echelons of melee combat, or study for months to reliably learn how to put more complex things together. The fact that the player can sit down with a book and suddenly learn how to put together engines reliably in the space of a day is therefore countered by the slightly faster skill degradation. That said as I've stated the goal is to greatly lower the skill rust rate, and have it only apply to your actual checks, not reduce your true skill rating (though you will need to work off skill rust before you can improve your actual skill scores), something that does happen in the real world (though at a reduced rate). After all I know that I certainly can't do plenty of the more stress/strain related engineering stuff, and I supposedly learned all of that only a year ago. There is a reason why continuing education is required in most professional fields after all.