It's not just about the tournament though, he's been making sensible choices since he took over. Like I said, whatever happens against Germany (it will be a very tight match), Deschamps will be appreciated like rarely a french coach has been in the last 10-15 years.
Not giving Martial more minutes isn't a mistake, I only see this spouted on Utd boards
. You would think Deschamps is preventing a Ballon d'Or from playing. The french fans, the medias, no one care that much about Martial as long as the team doesn't need and France hasn't truly needed him so far. Whatever he can bring can be covered and better at the moment with Payet and Coman.
THE mistake Deschamps may have made is not taking another striker instead of Schneiderlin because Gignac, despite his goals in Mexico, is just not up to the level anymore. Morgan is pointless, again what he can bring is easily and better covered by others.
Cantona can be known to have stupid opinions though.
Coman is ahead of him in the pecking order and like you said "You need your game changers to be ready/on form when they are needed." That and the fact that Giroud and Koscielny had yellow cards and could have been suspended for the Semi final. There is your explanation.
Martial will be the last option to play as a striker, Griezmann is still ahead of him when it comes to playing as a striker so he isn't truly Gignac's direct competition.