It's simple enough: If you don't use it, you lose it.

The more you have, the more there is to lose and the longer it may take. But skills and knowledge fade, just as easily as fitness, or a tan.

It's an insidious rule too. Insidious in the sense that you may not realise you're not exercising a skill. You may think that your just doing it slightly differently, but you will adapt. In whatever way you made your life easier, that's where you'll pay, in strength, fluency, depth, whatever.