What, you mean we really are good enough?

All the new evaluation schemes are coming into being, and instead of rating large numbers of teachers as ineffective, the opposite seems to be happening. And the 'reformers' don't understand why. Maybe it's that the bar is set so low for the benefit of higher student pass rates. maybe it's that VAM is total bullshit. Maybe it's that we just don't know how to define, much less measure "effictiveness". The NY Times had an interesting article on the subject, entitled Curious Grade for Teachers: Nearly All Pass, and it included this tidbit:
Grover J. Whitehurst, director of the Brown Center on Education Policy at the Brookings Institution, said variations in teacher quality had been proven to affect student academic growth. If an evaluation system is not finding a wider distribution of effectiveness, “it is flawed,” he said.

“It would be an unusual profession that at least 5 percent are not deemed ineffective,” he added.

So, you aren't really interested in finding out if we are effective. You only want to rank and sort us in order to get rid of the lowest performers (regardless of how good they might really be).
What, exactly, is the evidence for the second statement? Do we routinely fire 5% of our firefighters, police officers, legislators for lack of performance? Do 5% or more of doctors, accountants, or lawyers lose their license to practice their profession for perceived inadequacy?