I believe that Messi's performance will be secondary to the decision whether he plays the next World Cup in 2026 or not. With secondary I don't mean that it doesn't matter at all. But frankly speaking, I don't expect him to play a real mess in Miami. He will still be able to contribute to the team on a high level and have amazing moments for the crowd. That won't be the problem. But I think it will be important what the Argentinian crowd wants and what Messi's appeal as a brand and also as a supporting player for the team is. If the whole country of Argentina begs for Messi to play the World Cup, I am not sure that a coach would decide to publicly announce that Messi won't be nominated. The risk for the coach would be very high if things go wrong even when they would have gone wrong with Messi. If Messi does not announce his retreat, I think we will see him play one more World Cup.
Also, a lot depends on such elementary things as physical fitness and health. If all this is on the level, then I think there will not be a single coach of the Argentine national team who will risk not including Messi in the squad. But once again, I repeat that there are still three whole years left and a lot can happen during this time, especially at the age that Messi is at, when it becomes harder to recover every year and therefore some players on the contrary refuse calls to the national team in order to find strength and health to play for their club. I'm not saying that this will happen now, I'm talking about what may happen in a few years. The very fact of Messi playing in the USA is already an important factor in attracting attention to the 2026 World Cup.
I absolutely agree and it also depends on what Messi wants. Nobody knows yet whether even he himself would agree to play another World Cup. But when he now stays fit and shows some great performances in the MLS, I expect the fans around the globe to be wanting to see Messi play the World Cup or at least be part of the Argentina team. He will also feel the pressure that people want him to be there because he is their idol in soccer.
But yes it is three years to go and nobody knows what is going to happen. It is also possible that in two years he goes to Saudi Arabia and it is the Saudi Arabians who want him to play another World Cup in order to draw more attention to the Saudi league as well. A lot of potential scenarios and in the end we will have to wait and see how it plays out.