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Barcelona only got back to good last season because Real Madrid wasn't so consistent in winning matches, whereas previous seasons Barcelona didn't look as good as last season. So in general Barcelona are just trying to look good and that was what Barcelona managed to get last season, but for next season Barcelona still has to fight hard to be able to look even better because Real Madrid will also try to be better in La Liga and so also at UCL.
I agree with your opinion, so it's true that the rivalry of the two best teams from La Liga next season will be eagerly awaited. Real Madrid have lost their best striker to the Arab Pro League, so they will have problems to sort out if they are to win the title next season. While Barcelona has a many strikers, so I don't really think about their performance dropping despite losing Dembele to PSG.
The competition between Barcelona and Real Madrid will be highly anticipated because Real Madrid has yet to find a striker to replace Benzema. Although Real Madrid have signed Joselu so far, he does not appear to be the right answer to replace Benzema well. So I think next season is a good opportunity for Lewandowski if he still wants to maintain his top scoring streak in La Liga.