Currently, you rely on Nvidia to keep the drivers up to date and working with the kernel you use in your OS.
If the drivers aren't upstream, a future kernel can break them. So if you update your OS, it might no longer support your GPU without installing an alternative. Nvidia are responsible for getting this fixed in a timely manner.
If the drivers are upstream, then people aren't allowed to break them when they make changes to the kernel. There should never be a kernel or OS upgrade that causes the drivers to no longer work, the people who made the breaking change are responsible for fixing it before it gets released to you.