I don't necessarily think it was all about trying to scare wizards with a practical future threat.
It was hate mongering. He was just trying to bring out the irrational hatred that the pure-blood wizards already had toward muggles. Even if they logically know they aren't in real trouble, they're more than willing to jump on any reason to preemptively turn against them they can get.
I absolutely think it was Rowling's narrative on Trump and modern racism though; that's pretty apparent.
No, its been obvious even from the first book that Grindelwald was supposed to be some kind of WW2 stand-in. ---