Robert Lewandowski is currently above the age of 35 and as such, you don't expect him to continue be as prolific as he was during his 20s. Currently in the Barcelona team, they do not have any striker that's capable of challenging Robert Lewandowski for to no.9 role at the club and that why I don't expect him to struggle retain his place as the club's regular starter next season. The Poland international have been very impressive with his performances for the club and was one of the main reasons Barcelona win the Spanish La Liga last season. This season, he's be doing his best to help the club remain competitive despite the injuries that kept jim sidelined for some part of the season.
The club's current financial crises I don't think will permit them to sign a striker that'll be able to challenge Robert Lewandowski for the teams regular shirt next season so I think it's safe to say that we're still going to witness another season that the Poland international will remain as the club manager's first choice
Robert Lewandowski is a high profile player of which he showcase his talents while in Bayern Munich and just like you said age is calling on him now as you can't compare his energy and capacity at Bayern Munich to now that he's playing for Barcelona but however, regardless of his age he has still been a major player for the team's squad and on several matches he proved his worth so he should enjoy his playing time at Barcelona.
Apart from the fact that Barcelona are experiencing financial difficulties now but that doesn't mean that even if they were to sign a prestigious player that he gonna become the most preferred choice instead of Robert Lewandowski so let that be aside for now though some injuries he got during the season gave him some setbacks but he wasn't that off the game so let's watch out how next season will turn out for the Polish international footballer.