they wouldnt need a hard cap an alternative is to go with a deterioration of your skills that you dont use for example you lost 1 skill level every 3 days in game or something like that
I agree that this is much better than a hard cap.
If my character uses swords for a large part of his career, then wants to switch to magic for a challenge he should be able to - he'll learn slower because it's a minor/misc skill, but he should still be able to be the best conjurer in the land if he keeps it up.
But yes, doing that could deteriorate his blades skill due to lack of use/strength training.
I would suggest a cap of deterioration, though, to indicate the 'you never forget how to ride a bike' effect. i.e. Your major skills won't deteriorate, due to your natural affinity or years of constant training, your minor skills will deteriorate but never below 50, and misc skills will never go lower than 25.