The Argentina team will have to prepare the team without Messi and Di Maria for the next World Cup. Because the chances of both of them playing in the next World Cup are very low.
It's true that Messi and Di Maria are of course too old to participate in the 2026 world cup and that is an opportunity for other young Argentinian players to fill those positions soon. Maybe at this time with the existing Argentina squad it is still possible to continue moving forward, but the role of Messi and Di Maria as creative figures in the Argentinian attack line will be a serious problem if there are no replacements. Maybe in a year or two, new Argentinian young players will appear, coupled with the current squad they will try to defend the world cup title.
I think it is obvious that the Argentinian national team is facing a generational change because it is not realistic that their main players, Di Maria and Messi, will play in the next World Cup in 4 years.
Will new young players be able to replace Messi? I think that is impossible and that Argentina will have to change the style and concept of the game and play more as a team.
After the departure of Pele, Brazil had to wait 24 years for a new world champion title.
Argentina had something special and unique with Messi. Without Messi, they are just another team out of many.
Indeed, you should try to give up Messi as the team's foundation in attack, if they don't immediately get a strategy or new players who can fill the void, it seems that it will be increasingly difficult to defend the world cup title. I'm not sure if Argentina will keep the same performance in the 2022 world cup and surely there will be more and more senior players who might retire, not only messi in that discussion.