There is a reason Messina hasn't been hired by an NBA team yet, and my take is because he would be too much of a "project" for most NBA teams with a decent fan base that has a "win now" strategy (Knicks, Lakers, etc), therefore Messina isn't well suited as it would likely take him 2-3 years to really hit his strides (first learning how to adapt his strategy to NBA players, then perfecting his strategy, etc).
I think the Millers family is such a unique owner that would take a punt on someone like Messina long term, provided that Messina gives the Jazz Org. a long term commitment. I think they really wanted Ty to be that guy (when Ty was hired, Gail was nearly brought to tears when she said that she really wanted Ty to carry on the tradition and be with the Org. for a long time). Unfortunately I just don't think Ty has what it takes as an NBA coach. By all accounts, Messina definitely has what it takes, it's just whether or not he can adapt what he has to the NBA style/culture.