The OP mentioned that Bucks' slowness is their serious problem. The Mavs might not be the "perfect comparison" for the Bucks (is there ever one?), but there are similarities -- particularly in the defensive slowness of each team.
Tyson Chandler was a phenomenal paint protector that anchored the Mavs defense. Brook Lopez played that role pretty well last year averaging a career high 2.5 blocks per game. Giannis actually helps in that area too (much better than Dirk could).
Marion and Kidd were good perimeter defenders for the Mavs, while being slow each at age 32+. The same can be said for Khris Middleton and George Hill as not super fast, but still intelligent and solid defenders.
The Mavs certainly were a special team and you could consider an anomaly in terms of championship teams. But this is also a rare year where the NBA title is completely up for grabs. I agree that the NBA has changed dramatically since 2011, but the Bucks have done a great job at tailoring their team to today's NBA (stretch on offense, length on defense). I'm not sure they will come out of the East, as they have strong competition in Philly and Boston, but I wouldn't say they're in trouble. At least not for this season.