As excited as I am that Allegri is gone. I just can't bring myself to be get too excited for Motta. I'll try and remain positive but it's hard after seeing so many failed managers since my time as a fan.
I remember when we signed Del Neri off the back of him having a great season with Sampdoria while having them play quality football. He took them to 4th place...
Then he was absolutely awful the following season with Juve.
We had a poor mercato when Del Neri arrived too. Del Neri never coached a big club prior to coaching Juve and during his time with us, he most famously implied that Juventus are not expected to win trophies every year.
You are severely underestimating the importance of having spent time at a big club either as a player or as a coach. It's not the most important factor for a coach to succeed but it sure helps in their progress.
I am positive about Motta doing well because he has a plan and specific philosophy of how he wants his teams to play. Whether or not he can be an instant success remains to be seen but with a couple of strong transfer windows, I believe we'll eventually start to see results.
Just look at the progress Arsenal have made under Arteta. They would've won at least 1 league title by now if not for state owned City and their stacked squad.
You are seeing many big clubs heading in the same direction, appointing young-ish coaches with a view to long term success.