Well its not that the turbos are junk, but the fact that they are small, which goes back to the whole lag/twin turbo deal. Since they are so small, they spin a lot faster than a large turbo would. My stock turbos had over 100K miles on them and they are still fine btw because they were taken care of.
Thats the other problem, people cheap out on oil, and dont change it on time. Turbos also sit in your exhaust, so they get pwnd with heat a lot more than superchargers do.
Yes, the 2 things I mentioned above is why audi switched. They dont want to pay for turbo warranty work.
I will argue that turbos are a lot better than superchargers, but they require better maintenance. Which for people who aren't morons its not a big deal, but (especially when it comes to mustangs ) there are a lot of morons who buy cars.