Well,
You're assuming that the very last key tried is the proper one. If you're going in a random order (you know, instead of starting at like 00000000000001, with more zeroes before it), I'm sure somebody would find it before then.
The problem is co-ordinating it. I remember there used to be a program that was trying to decrypt the DSi common key, and it would check into a central server that had a list of what's been tried already... I used to run that thing, too, but it sort-of vanished... I had actually heard that some elitist hackers DID find the key already and just didn't tell anybody, so the rest of the world was just wasting CPU power on nothing. I'm sure somebody, somewhere, has figured it out - that's the problem with hackers, they don't want to come forward for fear of being sued, so they just do it for fun and never tell anybody.
Which is why we need this to be a group effort
But yeah. Brute-forcing should totally be possible, it was being done for the DSi too... we just need to make sure this time everybody can view the results, etc. I'm not saying it's very practical, and I'm sure a different exploit will be found before anybody finds it via brute-force, but it would certainly be better than sitting around doing nothing while waiting for some elitist hackers to be nice enough to share their findings with us. No, I'm not going to coordinate this, do not ask me.