The simple answer from my side to such questions is that everyone's personal favorite player is the best footballer in the world. It is probably known to everyone that everyone has a personal preference do everyone may have different opinions and bases, just like many people support Argentina so it is natural that they will choose Messi, just like many people support Portugal so they will choose Ronaldo. But personally, I think those who have played well in the past and are still playing well in the present and are well-known to everyone in the world today are all legendary players. We can never underestimate the players you mentioned here, we have to admit that they were or are very talented players at one time.
Lionel Messi or Cristiano Ronaldo both of them are legends and its hard to chose between them. If both Argentina and Portugal are playing against each other, i will be more attracted towards Lionel Messi but that doesn't mean i would not love Cristiano Ronaldo. It is just that I am more interested in the Argentina team as compared to the Portugal.
Even if these players become old, people will still love to see them in action and want them to continue playing for their countries and leagues for as long as possible.
In fact, in my opinion, these two players are the two brilliant stars in the football world. I have been enjoying the game of these two players for almost a decade, and whenever a footballer spreads a lot of excitement, the names of these two players must be mentioned. In fact, the Argentina football team is much stronger than Portugal for which you are choosing them. I see that many people do not like Portugal but Cristiano likes Ronaldo. No matter how much we compare these two players, they are not smaller than anyone, that is, they are a big player. At this stage I agree with you that even if they get older, people will watch their football match.