Who needs a reason to get a new bike! Isn't "want" a good enough reason? Well, I'm sure you and your daughter both will be thrilled when you get your new bike next year.

I'm surprised nobody mentioned that rider experience/ability also plays quite a major role in how long your components last. I remember back in college when I got into cycling in a serious way, some twenty-mumble, mumble years ago. My first road bike was a schwinn, with downtube shifters, but good components for the time. These would have been friction shifters, not the indexed shifting that is standard today. I went through a couple of drive trains and many chains learning to ride well. Even with indexed shifting today, newer riders have a hard time finding the right gear, grind them, hammer too high gears on hills, shift when there is a lot of pressure on the cranks, etc. The hillier the terrain, the more damaging, too. I found that after I became an experienced, competitive cyclist, even though I was riding more, my components lasted longer because I was naturally more gentle with them. So there is yet another factor of "user error" that plays into it beyond quality, maintenance, and riding conditions.

I agree with Indy... there's not a good answer because there are too many variables, including how quickly you learn to ride smoothly.