Well, someone would have invented it sooner or later, because after the discovery of nuclear physics it doesn't take that much brainpower to figure out the basic concept of "if we smash a bunch of neutrons in these unstable isotopes, then we're going to get a big kaboom!" The actual technological legwork is a bit more effort (and the Germans famously failed at it, although they probably would have gotten there eventually), but not insurmountably hard either.

This is the curse with a lot of these technologies.