I have this feeling that he might still be back at the next World Cup, but in a different position 
It is very likely that he is not going to play another World Cup, but looking at it realistically, what else should he answer? Perhaps he stays incredibly healthy, extends his contract and still performs for PSG and the fans want him, how will he decide then?
I think we should appreciate that he will be around for one more World Cup and then it is about following from season to season like we do with Ronaldo. They choose different ways to communicate, but I think Messi's way is less confrontative and might some day turn out to be beneficial.
As player, he is already announced this is his last world cup and with these hopes are high for better results from him and his side as they are already in top four with their quality and skills but what would be happened no one can predict because we have four years and in this time many changes can happen to him and related to soccer, so we have to wait for his next movement with most chances as right now things are happening in PSG he will stay there for few more seasons, and they will try their best for having Champions League title which is surely the most desired trophy for their management. It is very likely that he is not going to play another World Cup, but looking at it realistically, what else should he answer? Perhaps he stays incredibly healthy, extends his contract and still performs for PSG and the fans want him, how will he decide then?
I think we should appreciate that he will be around for one more World Cup and then it is about following from season to season like we do with Ronaldo. They choose different ways to communicate, but I think Messi's way is less confrontative and might some day turn out to be beneficial.
If his health and body allowed him then surely we can expect something positive from him for the next world cup which is going to play in the USA, Canada and Mexico but this all depends on because few years back he announces retirement and then comeback to play now this can happen again as currently he is fit and have enough quality for play some more years.