The good news is that the rest of the car has proven itself to be very reliable. Other than the occasional rear axle being out of spec, this car doesn't have any reliability issues you need to be concerned about. I'd say this car is fairly "bullet-proof" for what it is. That's one of the main reasons I like it.
As far as why they haven't completely fixed the rear axle issue at this point...
This is only my opinion. I'm assuming that not enough owners have actually complained to Mitsubishi about it. If they did, I would think there would be a technical service bulletin out by now. We have the most knowledgeable Mirage owners on the planet reading this forum, but I think we've only had something like 30 people actually check their rear alignment and pursue a replacement. I'm thinking that 97% of Mirages on the road right now have
never had an alignment check done. Why would you when it's a new car?
If you own a Mirage but you don't read this forum, you probably have no idea that your car may potentially have this issue. I'm still stunned by people on this forum who are on their 3rd set of rear tires...but their car has never had an alignment check?!?
Automotive companies are run by accountants...not engineers. Mitsubishi may have concluded that it's cheaper to replace the rear axles of the few people that complain rather than to spend a couple of million dollars re-tooling at the factory. Remember that Honda has had the same issue with the Fit (it uses the same type of rear axle as a Mirage)...and I'm not sure they have a TSB for their problem, either.
The important thing is that you have taken the time to read the information and understand the problem before you make a purchase.