Messi leaving is good for Barca long term:
1) Messi is 34 years old
2) He takes up an enormous amount of wages, his departure will solve the financial crisis
3) Barca can build for the future with the young talents de Jong, Pedri, Fati, Ilaix etc and some signings, no more "win now" bullshit
4) Barca can have a team of young players that can press, which is essential in modern football (Messi's work rate hindered Barca in the CL)
5) Some players can express themselves more and take on a more protagonistic role, Griezmann comes to mind. He likes to operate in Messi's spaces
6) His departure is putting pressure on Tebas and will pave the way for the Super League, which long term is what is needed for Barca. If current trends continue the PL and PSG will leave other big clubs behind fianancially