Thank you for explaining it to this newbie.
I'll use ETH as an example. I'm operating under the assumption that it's popularity (and hence market cap) is bound to rise faster than its difficulty. While mining rewards may dwindle as difficulty rises, the value of mined coins rises faster. Of course, we have no way to know for sure, but is my reasoning at least based on good logic?
Broadly speaking, you are right. Except your assumption may not hold true. If the price stagnates or even falls, your rig will quickly become worthless. Difficulty will likely continue to increase during a period of price stagnation. Even selling off the parts won't get you much back as ebay will be bloated with them. From a pure business risk/reward prospective, buying and holding the coins themselves is less risky and possibly more profitable in the long run.
But not as much fun as mining!
Don't be tempted to see it as more than a hobby.