Millsap is basically the love-child of Brandon Bass and Jared Sullinger.
Millsap is a better rebounder than Bass and he is closer in offensive value to Kevin Love than he is to Jared Sullinger.
He has career average of 16.9 Points, 9.1 rebounds, 2.6 assists, 1 block, 1 steal, 4 fouls and 2 turnovers per 36 minutes, which (other than the steals) are all right around where Sully's career numbers are so far.
Obviously the key advantages for Millsap are his defensive versatility and his outside shooting, both of which are significant upgrades over Sully. On the other hand Sully has an advantage of size and strength, being a good 2 inches, 20+ pounds heavier, and with a similar overall wingspan.
Now overall Millsap is obviously the superior player (by a significant margin) and nobody in their right mind would deny that. But you're talking about a spending max contract money on a 30 year old 6'7" PF who gives you 16/8/3...on a rebuilding team who's biggest weakness is lack of interior size.
Millsap isn't a dominant enough scorer to carry an offense, and he's not a good enough rim protector to anchor a defense - and at 30 years old he lacks the potential to develop either of the above. He's an outstanding 'jack of all trades' guy (much like Iggy) who could be the ultimate glue guy on a contender, but he offers a skill set that we don't really have a need for, at a position we're already stacked three deep at, for type of money that would eat up the vast majority of our cap space for the near future.
Throwing max contract money at such a player doesn't make a huge amount of sense for this specific team.