For sure better than him:
Jokic
Embiid
KAT
Bam
Sabonis
Gobert
Vuc
Probably better than him, but at least a discussion:
Capela
Brook Lopez
Myles Turner
Ayton
Nurk (if healthy)
After that though? I think you could probably slot him 11th-13th fairly comfortably. And what makes him valuable is you can fit him with star wings and not have your offense built around him like most of the top 10 C's. Center certainly isn't as valuable a position anymore, but what Holmes does on defense with his ability to hedge the perimeter while still able to protect the rim is an incredible valuable skill-set that's hard to find. Especially in a super productive offensive player (68% TS in last 71 games).
Assuming we flipped Barnes AND Buddy, I'd be willing to go 18mil a year. His skill-set is too valuable to let walk.
Thats fair assesment of Holmes' skillset. This is the second year where Holmes is (one of) the best performing King in these advanced metrics. Part of it is that he is a good player and does a lot of little things well. Part of it is that our team doesnt have superstar level players. Its easily arguable that around 10mil for him is a sum we should absolutely keep him long term.
My basic principle about big men is that unless you are absolutely elite at your position, I'm probably not paying you significant money or investing a significant draft asset. The reasoning behind that is that every year you have guys like Whiteside, McGee, Howard, Baynes, WCS (you name it) that can produce on a specific role for nearly a minimum.
Holmes extension comes a year or two too early in a sence that he is the type of guy you extend as a part of already great group. By himself he is not much of a floor raiser but as part of a group lets say like Fox, Hali and Cunningham supported by Barnes, he is absolutely a guy you should look to keep. Thats why he is a difficult situation, it all depends of the assumed market price compared to possible trade return for him.