We know we have unemployment i.e. there are people available that would take the job.
If you can't tell if a person A does a given task, in this case teaching, better than a random person from unemployed pool, the rational thing to do is to hire a random unemployment person for minimum wage.
That, of course, would be absurd: we all know that there are differences in performance and they can be measured and they are being measured.
As everything fuzzy in life, they are not always being measured perfectly, but that doesn't mean we should throw the baby with the bathwater by declaring that because we can show that sometimes the measurements aren't fair, they can never be fair. That's a fallacy and even article in question is based on assumption that you in fact measure the quality of teachers.
I, for one, know which of my prior teachers have been good and which haven't and most of my classmates would agree. Asking former students may be one idea although it's a pretty slow feedback cycle. You wouldn't be able to ask students immediately after the year is over since it would probably be correlated to the grade they received but years down the line people would have a pretty good sense of who the good teachers they had were.
One can also have former teachers monitor more as well as have more frequent observation sessions.
I'm sure there are tons of ideas people have and it's important to choose the best ones but none of this is going to happen until we make it a point to pay for performance.
In a way, it's similar to startups - you don't know what the final product is and you can always improve but it's important to start in order to get the feedback, learn from it, and iterate on the product.
s/preform/perform/g