Mario games aren’t very hard to make compared to Metroids… The focus of Mario is immediate action, the fun of jumping and bouncing around obstacles in one general direction with limited temporary upgrades. The controls are simple, the code is simple, the graphics are simple, and you can pick it up and put it down in short bursts of time. There is a standard formula for the gameplay, but the characters are silly enough to be thrown into any setting, so even a flappy bird remake could be a Mario fangame. That’s not to say a few of these aren’t incredible works of art, but most are not very deep.
The tone of Metroid is such that most minigame spinoffs would be inappropriate. A Super Metroid Maker just wouldn’t catch on – it’d be hard to build, hard to use properly, and the fun isn’t immediate. We are engaged by the subtleties of atmosphere and the gradual progression of level design. You wouldn’t know if a game was any good or not until you’re well into it, so there wouldn’t be enough time in a year to even sample 500 Metroid games sufficiently. Custom sprites require real skill to match the style and detail. It’s the difference between making a novel and sketching a colorful picture book. It’s not a fair comparison. That’s why it is such a newsworthy deal when a single Metroid fangame reaches a 1.0 release. It is a serious effort and a serious accomplishment.
As for your other comments, I still stand by this post. It’s only dead if we want it to be.