Barcelona isn´t all Messi but it´s not a coincidence that the clubs most successful era is with him as the key player. It´s ridiculous to think that Messi needed Barca to succeed, he was wanted by everyone ( remember the Gamper game against Juventus?) and with his playing style would have success everywhere. Sure Barca helped him become even better than he maybe would but an insane fast, dribbling and goalscoring winger would have success everywhere. What you say might be more true for other club legends like Busquets, Iniesta or Xavi who would have difficulties with their lacking physicality but not for the best player ever..
The Gamper game against Juventus was after he already passed through Barcelona youth ranks. Messi became the best player ever at Barcelona. Barca developed him. It is not like he was the best player ever when he was a kid and he had growth hormone deficiency is it?
That is what people don't get. He became the best player ever, arguably, also because the set-up in which he was developed was the right one. Without that setup, and that platform that was and still is Barcelona and La Masia, nobody knows what would've happened. And yes, there's a possibility that even a talent like Messi would've been lost. Hell, it's possible that his health problems would've ended his career at an early age. There are talents like Messi in the world that never make it because the elements do not line-up like they did with Messi. Barcelona is responsible for making sure they did line-up the right way.
People trying to imply Messi was destined to become the best ever and we were just lucky he chose us, or that we are nothing without Messi are funny. But I get it, many are Messi fans first and Barca fans second, so it's understandable to a point.
Believe it or not, talent can get lost if some things in their life don't fall into place the right way and at the right time.
