I don't think Messi is the only reason Zidane hasn't taken charge of the French national team, the FFF still has great faith in Deschamps abilities even though he failed to lead France to win the 2022 World Cup. If Messi retires, there is no guarantee that France can win the World Cup because this competition is participated by countries from various continents, apart from Argentina there are still many other countries that have above average squad depth such as Germany, Spain, Brazil, England and the Netherlands. Maybe Zidane wants to show his dedication to his country, even though currently his chances of becoming the coach of the French national team are very small, he is still faithfully waiting for the opportunity.
Under the leadership of Desham, France achieved very good results and their game in Euro 2024 is not able to cross out everything that they managed to achieve. Decsam’s contract is valid until 2026, so I think that he will remain the head coach for another two years.
It is necessary to draw conclusions why the attack of France was so weak, but it seems that in the last matches of the League of Nations they begin to correct this. Philip Diallo, President of the FFF says that they will trust Desham, so this is also an indicator that he will remain in his place, at least until the end of the contract.