Again we are talking about Cristiano Ronaldo and Messi, I'm sure these two players were perfect players for ways in any team they were playing but both players are getting old and specialty Ronaldo can't have a good performance anymore like he was playing in Real Madrid and after three years I'm sure both players will have even worst performance. We had even more famous players they got retired and other younger players could replace them there is no need to worry about these two players in the 2026 world cup.
We are talking about an event that is scheduled to take place after 3 years from now. In case of Ronaldo, I am 99% sure that he will not be participating. For all practical purposes, he has moved to Saudi Arabia and Euro 2024 (in case Portugal manages to qualify) maybe his last major tournament for Portugal. In case of Messi, I am a bit more optimistic. As the captain of the winning side in 2022, it is his responsibility to defend the trophy in 2026. So the chances are higher that Messi will participate in FIFA World Cup 2026.
I think the same, even if Messi had a drop on his performance due to his age or an injury, if I was the coach of the Argentinian national team and I was given the choice between bringing a player which may not even play or Messi, I will pick Messi, as not only he could always change a game with a single play of his, but when we add the leadership he has, he could easily help his teammates and give them some advice even if he was not on the field.
I don't know for sure whether they are still playing in the World Cup or not, because we all know that previously many speculated that the Qatar World Cup would be the last World Cup for Cristiano Ronaldo and Lionel Messi. Maybe we'll have to see how it plays out, because it's a long time before the World Cup is actually held. 2 or 3 years is a long wait, and we don't know whether they will announce their retirement soon or not, because the possibility is also very large.