These are still players that were bought from abroad at 17/18 for a few million each as top prospects. You'd expect to make a squad player out of them within a couple of years if developed right. It's the lack of players coming through the ranks, which forces the club to make signings like that or like Nolito/Vidal.
And it won't be as straightforward to develop Halilovic or Denis for Barca's team because they've come through the ranks at other clubs and aren't used to Barca's style. Going out on loan won't help that process either.