It is far more important to have open sources for all programs that are executed in privileged mode than to have the source for the programs that are executed in user mode.
The kernel can control what a user-mode program does and limit its access to any hardware resources or CPU time, but it cannot do anything against kernel modules that are executed in privileged mode.
So having the source for all the kernel modules is the main priority. With this new open-source kernel module from NVIDIA, the only closed-source kernel module used by a large number of Linux users is no longer so.
Now the main problems remain the BIOS/UEFI of the computers, which is free to override what the operating system does, due to the exceedingly stupid System Management Mode invented by Intel, and the firmware for the auxiliary CPUs that may be used for unauthorized remote management in most Intel- and AMD-based computers.