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.
the problem was the nouveau would do this before when firmware was easy to intercept from the driver (and could be extracted easily). the newer driver uses different methods of firmware upload and it's a real chore to do so now.
so nouveau gave up on it. they also expected nvidia to drop some firmware.
now that newer cards have GSP.bin firmware, which can be interfaced with easily - things are different. i would wager a guess that it's similar to atombios from amd. you just call a function in GSP and it knows what registers to poke with the right values to achieve what you need.
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.