Certainly an interesting question. I would love to know the answer. Do you mean that someone at Toho perhaps said Godzilla would definitely never, ever be re-invented? Or perhaps he would "just die"?
Yeah, pretty much all of these movies do end the same. I don't mind it at all either. I actually like it because the endings make it feel as if each movie has never actually ended and the place/setting are all in tact no matter how the next movie portrays events in the past. So it's not like Godzilla dies in each movie so, say, Godzilla 1984 could easily be ratified since in that movie's world the events from 1955-1975 never happened.