It sort of begs the question will opera still be relevant in a few years down the line as rendering on phones gets cheaper and faster.
It kinda does. With the technology they have right now, I'd take interactive-javascript-capable Opera Mobile over proxying Opera Mini any day. But if in a year, Opera Mobile can make, one way or another†, Facebook or new Twitter or Google-Wave-of-the-day or any modern website noticeably faster, I won't care much if it means my data is going through Opera's servers, just like I don't care my email is on Google's serve…
Take a look at the Turbo feature in Opera Mobile. It's similar to the Turbo feature on desktop. Once again, data is being sent through the server and it is being compressed. However, Turbo is capable of handling interactive-web-content (JavaScripts) because the rendering part is still being done locally.