Playing in arguably the toughest league , reaching successive CL finals and playing dream football with a group that could be together for the next 8 years..
You do know that their high action - high intensity football consumes a lot of energy. It definitely causes a lot of fatigue, you can't keep doing it continuously for 8 years with same players. Granted some of them might be able to, but its not for everyone.
It is going well for now, doesn't guarantee it can go on forever. Everything has a time limit.....
Also, if you think 4-5 years ahead Barca will have a completely new XI, none of the current legends will be here (bar maybe Messi), new leaders will be born, new legends will be made. If anything, its the right time to join Barca.
Winning PL is only going to get tougher, City is already up there, Spurs are closing in, Chelsea are unpredictable (you never know), Utd can do it, if they get their shit together(although not quite easy, I admit). Also its not easy to match 97 points in a season, so its not all rosy over there.