Obviously there are no guarentees in football. Likewise it is all about opinions and the game has advanced in so many ways since the likes of Pele, Maradona and Eusebio played, I'm not even sure it is fair to compare them to the likes of Messi and Ronaldo. There have been many great footballers over the years no doubt, how many of them were able to combine pace, strength and skill like Messi does? It is certainly open to debate as many would argue Puskas, Best, Cruyff, Zidane etc. all would be right there with Messi.. even though they played different positions and brought their own unique but brilliant contributions. It really is impossible to compare.
Do Argentina have a good chance of winning the world cup in the next 10 years? Yes they do, because they are one of the leading nations in the sport. You can make a case for them just as much as you can Spain, Brazil, Holland, Germany and England. All of these nations have loaded squads and importantly loaded youth squads coming through.
I don't necessarily agree that Messi needs to win a world cup to be considered one of the greatest, if not the greatest, of all time - because to win a WC takes a collective team effort, and while Argentina may have the best attack at international level - their midfield and defence leaves a lot to be desired. I don't see how Messi can do anything about that, no individual can be responsible for winning the world cup.