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.
Lewndowski is still a good striker. He was able to make 18 goals for the season. Indeed, he is not young anymore, he is 35 years old. Maybe for a lot of running he's not what he used to be, but for finishing he's still pretty good. Selling him is a good option if Barcelona already have another replacement. But hold for 1 year next season I think is also good. Lewandowski is in the top 5 scorers this season.
1. Alexander Sørloth 23 goals
2. Artem Dovbyk 21 goals
3. Jude Bellingham 19 goals
4. Robert Lewandowski 18 goals
5. Antoine Griezmann 16 goals
Source:
https://www.espn.com/soccer/stats/_/league/ESP.1/view/scoring