I was discussing the tradeskill system I wanted to use in this game I'm making and... well.. here it is...

Let's say you have high strength and low intelligence, and you're an accomplished blacksmith. Now supposed you suddenly decided to take on a Wizard's tradeskill, like alchemy or something. Now when you were smithing your skills went up fast due to your high strength score, but now you find that you have to practice a LOT just advance a little in alchemy. At the same time your smithing skill starts deteriorating. When you gain 1 point of Alchemy you lose 10 points of Smithing. if you try to make up for those 10 points of smithing you lost, after easily gaining back 9 points of smithing you lose that 1 point of Alchemy you just worked so hard to get.

The argument put to me is that your tradeskills shouldn't depreciate and should remain the same while you take up another, which will allow you to become a master of all tradeskills (which I don't want to happen). What do you think?

I agree. Skill depreciation shouldnt happen, at least not at that rate. Especially if the character is doing something to increase intelligence. I'd figure that the smarter you are, the easier it is for you to retain your knowledge, including skills and the like.

I agree. If anything, just put a cap on total points you can have in all trade skills. Let the player choose what to deteriorate if they want to practice more.

I agree. Instead, while you're working on a tradeskill that's hard to raise because of a lower stat the other tradeskill will gradually go down. You might start to notice the loss of your smithing skill points even if you haven't gained one point in spellcrafting (this is assuming of course you have high STR and very low INT)