The problem with Nouveau was that Nvidia hardware could not be reclocked (the cores were basically always in low power mode) without a signed firmware blob. That blob couldn't be legally distributed except by Nvidia, so the open source folks more or less gave up on a high performance driver.

Now, for newer hardware, Nvidia has changed some aspects of the firmware and allows redistribution. So it's feasible to make a good open source driver.

AMD and Intel also use different drivers for different hardware generations, since eventually things change so much that it's better to start clean.

With regards to reverse engineering, Mesa has a number of reverse engineered drivers. That isn't anything new.