What makes Lewandowski strong despite his age is his ability to be a team player. Not all the strikers have that ability, they can't put back their ego and instead still try to score that goal themselves, but they lack pace and precision and can't do it the same way anymore. Lewandowski in contrast usually never puts himself first at all cost. He plays in spaces in the box that others strikers don't see and he creates space for his colleagues when he knows it is the best course of action to score a goal. His effort to be the first defender on the field is still remarkable. He doesn't shy away from working hard.
Asides age, strikers like Lewandowski are rare to find lately so he still got more time in Barcelona maybe he might not feature in more matches if Barcelona gets a younger striker, but like i said, he still got lots to do at the club atleast pass his knowledge and experience to the future striker that Barcelona might probably sign, asides being a team player Lewandowski is a humble lad he's not as arrogant as most younger players lately and would always put his team first before himself and those are the qualities that would warrant the board to keep him for a while so the younger ones would learn.