As it turns out, the RGB AMOLEDs degrade much faster than the PenTile AMOLEDs, so the picture quality becomes worse faster. The case is with the blue pixels, as they degrade faster than the red and green pixels. The PenTile displays have less blue pixels because of the RGBG subpixel layout and that’s why they have longer life span.
You can check this image to clear your doubts. Pentile display is on the right.