This was a long time in the signs already of course and we really need fresh blood. Fans say luckily Real are also in decline, but we forget to respect others like Athletico and Sevilla.
In my opinion, it's not good and healthy when one team depends so much on one player.
Messi is probably one of the 3 the best players in soccer history but he is not young any more.
How long he can still play?
What will happen with Barca when Messi retire?
Who can replace Messi?
Yes, Sooner or later Lionel Messi will retire and most probably what is now happening for Real Madrid after Cristiano Ronaldo will happen for Barcelona too. But I am sure that effect won't be permanent. Barcelona is a big club with great academies. I expect them to manage to find an alternative. But it will probably take some times.