When did you change or upgrade your original model?

I don't think you can outgrow your device, assuming you got the right one. A model that leads you to Super-Os once should be able to do so again. The reason to get more models, to me, is mainly to see if another one works better than what you already have. Although they all feel a bit different, they pretty much all work on the same principles. Even if one is all you need, you'll probably want several different ones, just because it's fun. 

When I first started with Aneros, there was only one model, now known as the MGX classic.  I bought the next model and the smallest of any subsequent models, the SGX, now discontinued, shortly after it was released.

No, you don't need to continue to up the size.  After nearly 20 years, the SGX is still one of my top 3 favorites.

Variety is the spice of life so having options is always good.