Allen isn't nearly as bad as you make him out to be on that end.
Individually Nurk and Allen both are solid starting caliber NBA players. Both are quality fourth and fifth options. But then you have to look at how they fit in with a big 3, that already plays out of position.
It's one thing if the team was built with the right stars. We've built a team with 3 SG type stars, even though Durant is fine at the 4, Book and Beal should be SGs. Naturally we don't really have a point guard that can take advantage of all the tough big screens that Nurk can set.
Grayson fits in fine on offense around the big 3. Then defensively? He's your third SG match up wise. It's tough to play defense when your perimeter unit is all SGs.
Shouldn't be lost in this, team may have won, but they were also down big. They shouldn't be down that big to begin with. The bottom line is that this team isn't that good. When a team isn't that good, players like Allen and Nurk are expendable for better fits.
Here's the holes I see (Knowing that Beal, Book and KD are going nowhere).
Nurk, Metu, Okogie, and even Goodwin provide some defense. However they provide no spacing. No spacing in today's NBA = crappy offense.
The players that provide good spacing - Allen, Gordon, maybe a Bol Bol, they are simply non-impact defensively and don't fit in. In the case of Bol Bol, he actually has 2-way potential, but he's never healthy on a consistent basis and looks fragile (doubt he can survive a deep physical playoff title run)
Sure Allen can be part of a title team's core. He proved it in Milwaukee. But the main difference is our core isn't balanced. Way too many SGs. Way too many non-athletic defensively challenged offensive players. That's why for me Allen is not an untradable piece.