Let me see Messi score 86 or win Ballon D'or on any team other than Barcelona I'll say he's the greatest ever.
You don't ever have to call him the greatest ever because it'll always be arguable, but yours is another fallacious argument: that players need to "prove" themselves in other leagues. As if the greatness of his talent isn't immediately obvious to anyone who watches him play, and understands the game. Or that being loyal to the club you grew up at isn't admirable in itself. By your logic, someone should tell Franco Baresi he can't be considered the second greatest defender of all time because he stayed at Milan. I'm sure he'd agree with the rationale. And his old teammate Maldini, too. Oh, and Giggs and Scholes - their legacies are in question now too. And let's not forget Tony Adams and Totti!
LOL. It's ridiculous. We're watching the player/talent of his generation. There was Di Stafano in the 50s, Pele in the 60s, Cruyff - 70s, Maradona - 80s, Zidane/R9, and now Leo Messi. No need to attempt to pick him apart before his career is even over - just enjoy it.