It seems like you are too sure and really believe in Messi performance, but that natural because so far it can be acknowledged that he is great player and many people really make Messi their own admiration for the figure of football player they idolize.
Apart from that, only Messi, who for several years has always been competitor for Ronaldo, who we consider to be the best player of all time in his era, but sorry, I didn't think about Messi that far.
Maybe he will still be core player for Argentina if he really takes part in strengthening Argentina for the 2026 World Cup, but I not sure that in that competition Messi will still be able to play well according to exexpectations.
For Argentina itself is certain that they will get tickets for the 2026 World Cup competition or it could be said that Argentina has guarantee of qualifying for the 2026 World Cup.
I am not entirely sure like that, because in 2026 he will be quite old, not that he wouldn't be good, but he would be too old. On top of that, he is not playing in anywhere competitive anymore, both Ronaldo and Messi decided to go to leagues that are so much worse than the European leagues, which means that they are playing against teams that are much weaker, and that would make them go worse.
When you are playing against much better opponents, you try to be better, you strive to be greater, but when you are "good enough", then you stop working too hard on your craft. This is why I believe that if Messi keeps doing this for 2 more seasons, then we are going to see him both older, and less caring and that would not be a smart way to go.
For Messi, I don't know what he will do and what it will be like when Argentina takes part in the 2026 World Cup competition, but what is clear is that I won't have any hopes for Messi considering that he is not player that I really idolize.
But for Ronaldo there is still the possibility of participating in the 2026 World Cup with Portugal and if he plays in small team, I don't think so because his mother had asked Ronaldo to return to defend one of the teams from Portugal, namely Sporting, this will be the final days of retirement for Ronaldo.
Yes, 2026 means there are still 2 season ahead and of course there will be many unexpected things happening later, but they are players who are considered great and important for their country so it will still be possible to use them.
I know that their performance will definitely experience very significant decline and in the future they will be left behind by the young players, perhaps their playing style is just running forward looking for position and waiting for pass to come in to be converted into goal.