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.
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.