Why Money Heist Season 5 won't be having a happy ending?

Netflix Money Heist season 5 will not be released sooner than September 2021, so you have enough time to think about what the series creator, Alex Pina said about the show's ending. Season 4 ended with inspector Alicia reaching Professor's hideout and pointing a gun at him. Many fan theories emerged after this. Some said Alicia might kill Professor while others said she might join the team. However, talking about the series finale, season 5, Pina mentioned that it would be disappointing for some but they will be providing justice to characters. Okay, we must be ready to receive some unpleasant surprises from the 5th season of the show. What would be a disappointing end to Money Heist according to you?