Lewandowski is a striker who so far I still consider a productive center forward in La Liga. He has scored 18 goals and 8 assists in La Liga this season in 34 appearances, certainly not bad for a center forward who will be 36 in August. Lewandowski is desperate to stay and become Barcelona's main center forward, but ultimately he may face competition considering Barcelona must also prepare to lose him in the coming seasons.
Barcelona has 2 other center forwards in their squad, they are Vitor Roque and Marc Guiu. The two players I mentioned are still young and have little experience, but Xavi sometimes gives them minutes in some matches so that Lewandowski can play better. Of course Barcelona needs a younger, stronger and more productive striker, that's because I'm sure Barcelona can't rely on Marc Guiu and Vitor Roque next season.
Lewandowski is still the best striker, but he is old. In my opinion, it is better for Barcelona to sell him because he still has high value because by selling Lewandowski, Barcelona has the capital to buy other players. Moreover, currently Barcelona is still having financial difficulties buying new players to strengthen the squad next season.
In fact, there are several players in Barcelona's main squad who still have high selling value, perhaps one of them is Frenkie de Jong. He also had to be sold because he had been at Barcelona for five years and had not made a big impact at Barcelona. Maybe this is what Xavi has to do so that Barcelona continues to move forward, by selling several players and bringing in new players because they cannot just rely on young players like Vitor Roque and Marc Guiu. Thus, I am sure that next season Barcelona will be able to compete in Laliga and the UCL League.