Except you said, "all the military could've voted in person" which is simply not true.
Again, the US military is stationed in about 150 countries around the world. Not just the countries that we're currently at war. Trump can't bring all of them home, even if he wanted to do that. Not even to vote. And, even if he could bring everyone home, disrupting and uprooting the countless defenses we have employed around the world, the military don't just station every person where they want to live. Again, there are military personnel in the US who get transferred across the country, at any time, and might not have the time to register in the state they're stationed. And even if they do register for that state, it could take up to 5-7 weeks. Leaving anyone transferred the month before election day out to dry.
So, mail-in voting isn't going anywhere.