I mean, lets have a look at the Nuggets. Their starters - Joker is a MVP level talent. Murray a second option all-star level player. Pope is a starting level two way player. Their 2 other starters are talented two way players that can go off for big numbers in Gordon and MPJ. They don't seem to have a hole in their starting lineup. You can look at other better teams, same thing. For the Suns to contend at that level, they need to be able to put forth the same starting lineup game in and game out, and be able to have that core lineup where there isn't a big hole.
Perhaps Grayson or Gordon will make it as such. Because certainly Nurk, Durant, Beal, Book are worthy of being a quality lineup together. However until we don't get that 5th piece that makes it consistent, we're gonna keep seeing this.