It didn't fail under Pardew though did it? It was only after he left it fell to pieces when we invested. Its not why we went down either. There are many ways to skin a cat.
You could have a great first 11-13 like Pardew, then suffer a drop off when you get a few injuries. The points amassed when at full strength was always enough, and indeed this process delivered our best results between Bobby's departure and the present day, Rafa included.
Or you could have a more consistent level throughout the squad like Rafa, but lack that extra bit of quality. You don't suffer such huge drop offs in form, but equally you can't reach the same levels of performance. Again Rafa was, like Pardew, very successful in achieving our annual goal of survival.
Relegation never came about because of the players in our squad. We have never under Ashley had one of the worst three squads in the league in any season I can remember. However he is the king of bad appointments. McClaren was an absolute horror show of an appointment. The previous time we had about 4 managers in one season and even placed our hopes on our famous number 9 who had zero football management experience.
With us relegation has sadly been due to bad management. Ultimately the responsibility for that lies with Ashley. The lack of progress from just surviving relegation is where Ashley and investment also have questions to answer - that is down to the players in the squad. There is no effort towards continuous improvement. All we do is try to stand still.