I believe Toho would have to file for bankruptcy during 1971 and Gamera would be more successful... Then we get to the Heisei Godzilla series. After Gamera vs. So and So Godzilla would reboot itself to be the Godzilla Heisei Trilogy. 3-4 movies would be made while Gamera might be getting an American reboot, which would not receive well. Gamera would reboot itself from the American film and it would end in the early 2000's while Godzilla comes back sometime after Gamera ends and Toho akes their final take on Godzilla, never to be seen again (up to 20XX).
Discuss other theories you have.