Actually, Barcelona doesn't need to buy players from outside. If they are willing to look for La Masia and give confidence to young players, then Barcelona can be successful without having to buy expensive players. Indeed, as you said, buying experienced players is important to accelerate Barcelona's strength because working with young players is difficult. However, the problem is that Barcelona is currently experiencing a financial crisis so using La Masia players is the only solution.
I think Flick's first season will help barcelona out of financial difficulties, and next season he will start to win a lot of trophies with barcelona.
Yes, it's true there is no other way but to use young players because the financial crisis they are facing makes it impossible to buy new players and of course giving young players the opportunity to play in Laliga, this is an opportunity for them of course they won't waste it either, Hansi Flick is in charge to educate them to become great players as well as develop each player's abilities and also set strategies in facing Laliga opponents next season, of course the competition will be even tighter.
Madrid as their rival will certainly not allow Barcelona to be at the top of the standings and rise to become a strong team in Laliga next season. Mbappe's arrival will make Barcelona's path more difficult, but Hansi Flick needs time to get Barcelona out of this crisis, at least having lots of young players. talented and also productive so he can become the most expensive player and make Barcelona's financial crisis better, I think Hansi Flick should focus on preparing for his debut match.