Some words are poorly designed and IMO that's one of them. Sure, you can just make up words and give them whatever meaning you want, but it won't work so well if the word itself causes a bias of assumption towards another meaning, especially if it's the opposite of what you want it to mean.

Just like inflammable. "In" used in that context usually means "not". Whoever decided that it should mean "very" in this one case was IMO a bigger idiot than anyone who assumed it's opposite meaning afterwards. Either that or an asshole if it was deliberate.