One, fbcon is not key to having KMS. It's just that KMS drivers provide it. But they could just as well not. KMS is an implementation of modesetting in the kernel. Nothing less, nothing more.
Two, the blob doesn't have KMS support, I never said that. It has an implementation of modesetting in the kernel. A different one from KMS.
So linux kernel modesetting support (separate from the nvidia blob kernel modesetting - how do you differentiate between the two -- prefix one with nvidia and the other with linux?) doesn't require having a built in fbcon? Hmm, seems like multiple phoronix articles point to this as being a feature of kernel modesetting in the linux kernel. Do you have a source that mentions fbcon is optional with the linux kernel-based implementation (not nvidia) of kernel mode setting?
If by "KMS" you mean the standard mode setting framework in the kernel, the blob will never be able to implement it due to license issues.
So since nvidia has their own kernel mode setting implementation, then I suppose they could just pretend KMS (linux kernel-based - not nvidia-base - kernel mode setting) doesn't exist and implement their own proprietary fbcon driver and accomplish fast VT switching. Then whatever licensing issues that stops them from using KMS can be bypassed right?