Again, Messi just turned 26. If he wants to, he's got at least three World Cup cycles left in him before he starts to decline. That's three more rounds of friendlies, qualifiers and actual World Cup matches to add to his already impressive total. While he's got a ways to go to catch Gabriel Batistuta's record of 56, it's safe to say he's got time on his side.
He is already tied for the second-most goals in the history of the Argentina national team (35 in 82 appearances).