Regardless is a tough word.
You are assuming that Mats will play and that he will be awesome in the current, his 1st Season.
Again, what if he will make a lots of rookie mistakes, or mistakes because he is not too familiar with La Liga and because he doesn't know the language?
What if more likely options for the current Season are:
1. we play Bravo, an experienced La liga Gk, and we win La Liga
-- but we will probably lose Mats in the future
2. we play Mats and lose a title to Real by 2-3 points because of Gk's mistakes
-- but we will get a long-term Gk for the future
Neither answer is "the right one".
But again, we are Barca. Current titles and the current team is a No1. The team is more important than any player.
Would it be fair to lose a title (fair towards Messi, Ney, Suarez and other players), only because we will get a Gk for the future (who may, but who also may NOT turn into anything special in the future)?
Also, what if Messi decides to leave because we are not winning titles anymore? Or Neymar, for example?
There is also an option to play Mats now, to lose the title, plus that he won't turn into anything special later even though we forced him to play, and even though we lost the title because of that.
A lose-lose situation.
Again, try to look from Lucho's position now.
If he will have bad results, he will be sacked after one Season.
If you have only one chance, only one Season as Barca's coach, would you play with the safe bet (Bravo currently), who is playing very well for now.
And that way you have decent chances to win the La Liga title.
Or, would you play Mats (because he is "the future") and possibly ruin the current Season, lose one more year in Messi's prime, and in the end, you would probably be sacked as a coach?
When you put everything on the table, is it worth to risk everything REGARDLESS on how Mats will perform?
Again, I also love Mats, but Barca, titles and the whole team are much more important than a single player.
Especially if he is yet unproven player in La Liga.
He will have to wait for a chance and we will see what happens.
If Mats leaves, we will buy De Gea or someone else in the end.
It is not like we will lose all good GKs in the world if one player leaves the team.