I am a huge He-Man fan (I could have paid off all my student loans by now if not for Mattycollector(!)

) but I need to dispute a few of those assumptions. Transformers was comparatively a much more relevant property than MOTU before the Michael Bay movies. They had ongoing cartoons, ongoing popularity in Asia (a very important source of cash flow for any given toy property) and new toys still in production.
Scooby Doo was also still being broadcast in reruns and still quite popular among the segment of kids who don't like combat-based adventures (and there's a lot of them, think about who buys Archie comics). Speed Racer you are correct about, which is why the movie bombed.
And even though I am a big MOTU fan, even I have to admit from a business perspective, He-Man is a very, very difficult property to sell to a producer. The fact that Conan the Barbarian and John Carter both bombed does not help matters.