I agree with many of your points, I've been here since '09, but I've been playing for many more years than that would imply.
I've made many single player maps and you're welcome to try them.
Why do so few people play this game?
I'll tell you.
PHONES.
No one likes to sit down at a PC anymore. Computers that play the old games are old, and don't work so well anymore. Consoles rule gaming now, but are quickly losing ground to mobile devices because people don't like committing to anything. They download a game spend a bunch of money to improve their character until they're OP, and then they lose interest because there's no challenge. After that, they move onto a new game.
AOM is an old style game.
Nowadays, even PC and console games are made quickly, released early, and patched later.
You pay $60 here in the U.S. for a new console game, three months later you pay another fifteen for the first DLC, and then three to four months later, another ten to fifteen for more DLC when it comes out... and next thing you know, you've spent a hundred dollars for a game you can't play in six years when the new console is released.
Old games weren't like that.
People love updates these days.
So people like me who can't afford new games, and dislike updating everything all the time, still play the old games. Age of Mythology, Age of Wonders, Dragon Age: Origins... etc.
Just put my original XBOX in the shop to get the tray fixed so I can play HALO2 again.
Would you do that?