If we still had Cousins, then it would have made total sense to target Millsap to add to the team at PF. Imagine a front court of Rudy, Millsap and Cousins - that would be pretty damn good. However, we don't have Cousins anymore.
Next season we will have a rookie PG, a second year shooting guard, and a third year center. There's still a question mark at small forward and if we signed Millsap, that would send Skal to the bench. So even if we brought back Rudy to play SF, I'm not convinced that starting five would get us to the play offs. It might, but there's a chance we'd be in the same tier as Dallas (ie not good enough for the play offs, but not bad enough to have good lottery odds).
For me, I'd stay away from Millsap and let Skal grow as our PF. Use our cap space on a starting SF. I'd go after Otto Porter or one of the SFs available with enough upside and age on their side to be part of the same team long term with our young players. There's no point adding a 32 year old all star player, they aren't on the same timeline.