Messi would be 39 by the next world cup and he is yet to make an official statement as regards if he will be playing in the world cup or not but then people are still speculating his appearance in the next world cup and hoping that will be his point not retirement, but then I think Messi would really want to participate because he may be too old at that time to participate in the world cup games but then if we see him participate then that's fine but if not it's not a problem too.
I think Messi will still participate in the 2026 World Cup but he will not be Argentina's key player at that time. I think Messi will have a very big influence on Argentina's young players. I think the existence of Messi, who is considered equal to another Argentinian football god, namely Maradona, will really increase the confidence of Argentine players and will increase their winning mentality. Messi's performance may decline physically, but mentally Messi is the best for Argentina in the 2026 World Cup.
Messi is still in a good form and I think that 2026 FIFA World Cup will be his last of his career but who knows what can happen in such an unpredictable tournament like the World Cup,remember the final of 2022,France had not shot on goal until the 73th minute and Argentina was comfortably leading yet in the final stages of the game they even risked to lose the cup.The teams will surely be better prepared as the weather is different in USA,Canada and Mexico compared to what it is in Qatar,the heat surely must have made some players not give their 100% so I think Argentina can still rely on Messi for another last World Cup and hope for the best.
He can be substituted but I doubt if he will be able to play 90mins but then he will be better if he is in the Coaching crew assisting them with some good ideas and tactics, he will be better with technical advises to the team of coach for his national team than he would be on the pitch, his experience will be a very useful contribution to them if he should be advising them.
Although if he happens to play it will really be a boost to his team mate and probably help their confidence but then I don't think he should as he definitely not going to perform like he did when he was much younger.