Ansu Fati looks like a great fit for Brighton, he can pair up well with Mitoma and other very good players from the team like Danny Welbeck, Kaoru Mitoma and Solly March to improve their attacking strength. There is no option of a buy in the loan deal, so he will return to Barcelona after the loan spell, or maybe his performance will impress other premier league team and that they will then try to tempt Barcelona to sell him. The biggest fear I have for Fati on this his loan move is that he does not get injured, because that is something that he is prone to.
Ansu Fati's arrival at Brighton will actually make their strength even better and their front line sharper. By relying on Danny Welbeck, Kaoru Mitoma and Solly March, there are several options for the coach to play them on the front line. The only worry is that Ansu Fati is unable to avoid the injury that stopped his career during the loan process. The English League is much tighter than La Liga and strikers are very vulnerable to injury when playing because they often experience physical contact, especially if the typical striker often accelerates when carrying the ball and this important point must be avoided by Ansu Fati.
Barcelona doesn't want to let go of their young players by selling under these conditions, so a loan is an option to maintain Ansu Fati's playing quality and when he experiences major changes in the Premier League he can return to Barcelona. So there are several considerations that make Barcelona not sell Ansu Fati and they still have hope that in the next season the player has developed far from now.