I would guess that it's roughly in the same ballpark as non GPL software being able to run on Linux. Just happens to be ring0 rather than ring3.
I don't think it is realistic to expect all drivers to be Free Software especially specifically distributed under GPL. A huge number of Linux drivers are GPL anyway.
What makes you so confident that Linux hackers would have managed to reverse engineer all nVidia's stuff to the point of having a good stable performant driver? Especially considering nVidia release new cards with new capabilities (thus requiring driver modifications) very frequently.
If nVidia had decided that they would release no free driver and therefor no driver at all, I don't see the business case for somebody being paid to develop a free software driver for nVidia cards and it's certainly not a weekend project.
The problem is that all the ARM-system chipsets are shipping binary-only drivers for Android and OMAP devices, even though Linux is used almost exclusively on those devices.
That's the business case.
nVidia may not care about Linux because it's a tiny portion of their market, but chip manufacturers (like PowerVR, for example) would have had to release Free Software drivers if Linux hadn't set the binary precedent. This would also be true for a lot of other hardware now. But unless there's a radical change in policy, it's too late.
What's the point of having a Free Software OS if you can't run it on your hardware because the closed-source drivers won't work?
I don't think it is realistic to expect all drivers to be Free Software especially specifically distributed under GPL. A huge number of Linux drivers are GPL anyway.
What makes you so confident that Linux hackers would have managed to reverse engineer all nVidia's stuff to the point of having a good stable performant driver? Especially considering nVidia release new cards with new capabilities (thus requiring driver modifications) very frequently.
If nVidia had decided that they would release no free driver and therefor no driver at all, I don't see the business case for somebody being paid to develop a free software driver for nVidia cards and it's certainly not a weekend project.