Barnes and Buddy are at the age where they should be starting to play their best basketball, and they are. Overall, I don't think anyone can point to them as being why this team can't get over the hump. Bogdan's regression is well-noted. Bjelica and WCS aren't starting caliber bigs. Yogi's an average backup. Fox, Bagley, and Giles are ahead of the curve, but they still have a ton of experience, skill, and/or strength left to gain.
There's a lot of room to grow, but I don't really agree with the concept that all these players are good, that we shouldn't tinker with it and let them grow. No, there's a good 5-6 man core here but this team is far from complete. They're not going to just waltz into the playoffs just by Fox, Bagley, and Giles' natural progression.