it explains how alchemy works
The gate flow is part of that explanation, which is the crux of my argument.
it explains a hell of a lot of other things.
So many things you couldn't even think of one to add to your reply, lol.
why does being able to use alchemy suddenly make the use of the real world impossible? that makes no sense.
Well, one of the things that FMA did try to explain is the history of both worlds, and alchemy gaining prevalence in Amestris in place of hard science is why it didn't turn out exactly like Earth. That's a core tenet of the series. Going back on THAT would be an actual plot hole.
"Oh Amestris and Earth turned out so different."
"Why? They can both use alchemy."
"Oh uh, good point."
If alchemy was allowed on Earth, by the rules FMA established, it would be exactly like Amestris was. And then it wouldn't be Earth, and then using it would be essentially pointless.