In order to fully understand the plot points, you have to understand reality of time itself. Time on this planet is measured by the rotation of the earth around the sun, and also the rate at which it spins, so there is that factor. Also, our fates are in a constant state of flux, or change, by the knowledge that becomes available to us and by the decisions that we make. (My great great granddad missed the Titanic by 20 minutes. Had he NOT been late, he might not have lived to reproduce, so I would not be typing this today.) 30 years ago I might have died from the abscess which these days was able to be diagnosed with a CAT scan and treated with the right antibiotics, so, I am alive today where I might not have been had medical science not been advanced like it was. We are also constantly travreeling in time, that is, forward. So any change that was, like as early as 5 minutes in the past, has the potential to affect our entire future.
The mere showing up of the Futurians already altered the flow of the time-space continuum. "We're here to kill Godzilla". BAM. Compounding this is the agreement of the human beings from the 20th century "Great! We're in, let's DO this." And all without any assurance that it was the right Godzilla, or the ONLY Godzilla. I'm of the opinion that G 1954 was left unaffected and it was the second Godzilla which was beamed to the Bering Sea and this is what turned into Godzilla 84.