All football players will experience a decline in performance as he ages, Messi has already passed the peak of glory during his career as a football player. Now he is looking at the remnants of his time as a player before deciding to hang up his boots. He is a bit lucky when compared to his rivals, PSG can give Messi the opportunity to continue to lift the title at the end of the season. While Ronaldo is struggling to get out of Manchester United, but unfortunately there is no club that is interested in using his services.
Messi was a great striker in Barcelona and scored a lot of goals, but after moving to PSG, he began to score much less, a lot of time between this transfer did not pass, he did not become much older. Therefore, I think that it is not the player, but the team. And if Messi could now move to another club, perhaps he would return his efficiency, and in PSG to realize himself in such a big rivalry not ease.
Messi wasn't a striker, he scored a lot of goals but a striker he was not. He is mainly a winger, or plays behind the striker, at the "10" position famously. You could check his positions in football manager or fifanad you will see that he is mainly a RW but could play at AMC position as well (attacking midfield center) and that is where he shines.
In PSG he played in the right wing as well, but he couldn't get into the middle like he normally would. That is not based on him or anything, that's based on how he is not used to playing with the other players there and the other players there weren't used to him, the more they play together the better they will be.