[Personal opinion] The 5K+ is not better than the 8K. The problem is that the 8K is below the expectations and I think Pimax can correct its blurryness by software

[mimaximax]

Here is Pimax recent (official?) statement about the matter :

@PimaxVR

The main reason why 8K need more demand than 5K+ is: 8K need rendering higher quality to do the compensation for the sharpness. with rendering quality set to 1.75 in the PiTool and the newly tuned gamma, the overall sharpness is competitive with other VR headset.
But the sharpness of 5K+ is still the best one in current VR market. better than vive pro and much better than others.