I didn't say they should all be replaced but none of them are irreplaceable and all could be upgraded. Even Markieff while performing nicely in the 6th man role has not demonstrated the ability to be a good starting 4 in the NBA. Also the creepy thing he has with his brother is problematic. If the Suns bring in 3 first round rookie bigs Marcus could be the odd man out.
He hasn't really gotten the chance this year. I don't really count his first two years because some players just take longer to develop. He was, at best, below average as a starter prior to this year. But he has improved a LOT this season and would probably do much better as a starter than his previous seasons.
I'm not saying he'll ever be great but he is coming along pretty nicely this season. He's already developed into one of the better bench players in the league. I wouldn't replace him unless you can do so with someone proven.
Edit: I know that the "PER 36" stat is not totally reliable, but he's at 18/8. With his play this year it's fairly safe to say that he'd be somewhere near those numbers with an increased role. I wouldn't call that
highly replaceable.
I just think it's way too early to give up on him unless you can find someone proven.