He wouldn't be playing for Cardiff reserves if he was more exciting than Januzaj.
And Pogba would surely have gotten more playtime if he had any promise as a CM, right? Also, Dæhli has gotten many minutes for the first team, and he's always on the bench.
Solskjær was the main reason for Mats leaving. Had it not been for him, then he might have stayed and gotten some playtime this season along with Januzaj. We'll never know. But I'm fairly confident that Mats didn't want to make the same mistake Eikrem did. They are 4 or 5 years apart, but Dæhli already looks much, much better. Going to Norway to play professional football and then to Cardiff, was a genius move.
The reason I think Januzaj managed to break through is because of his skillset. It's much easier to throw Januzaj into the mix because of his speed and dribbling. Mats plays more with his head, which is typically the sort of game you'd expect from seasoned pros. When it comes to young players, it's easier to go for the quick and unpredictable flair players and hope that they mature over time.
Overall I think that Mats has a more rare skillset, which will take him far. Quite possibly further than Januzaj, though it's obviously not a competition.