The thing that worries me more about Messi is the fact that he shows no sign of "trying to improve". When he was undisputed the best player, it was ok to not make himself better. But right now he's just the same player, doing the same things over and over.
This stamina issue has been discussed a lot and it's impossible that Messi can't improve in this aspect. I'm studying wellness in university and there are a lot of researches about physical stuff in football. It's normal to have a physical downgrade over the years but it's impossible that Messi is the only player that looks like an old guy. Every day a new method to increase stamina, acceleration, explosiveness, intensity are discovered. But no, in Messi's case it's the same "he can't run because he will get injured bla bla bla bla".
How about going to the gym more often? Build more muscles? Muscle resistance is something that can be improved with anyone, from kids to old people.
Messi can't expect to do the same damn thing and get new results. Right now he is in his safe zone, too relaxed. Do something new, motivate yourself, make yourself better.