@Bagheera: The original was deader than dead in 1954 when Dr. Serizawa used the Oxygen Destroyer, but that never stopped other members of the same species from turning up and doing what they did best.
Edit: This could also be a time loop, like the Godzilla that showed up in singular could be from the future where humanity has failed and is desperate, so they sent it back and all the other monsters in time for their ancestors to deal with. The one that will now show up could be one of the early Godzilla, who the doctor now wants to kill using Mecha and the reason he trapped himself in the computer was to learn how to kill Godzilla's from the future people, but him killing the younger generation Godzilla means the loop won't complete, and it needs to complete and go full circle or there will be timeline consequences.
Given the nature of the time looping in this series, this idea sounds very plausible indeed!
And don't forget the moths that turned up. That, to me, is a straight-up indication that Mothra herself is heading this way.
With all the talk about singular points and the catastrophe, I'm guessing that they're going to discover that they've ignored the tendency for singular points to cross planetary
ecosystems and become star-system wide problems, and that will be a prelude to the Singular Point version of the interplanetary three-headed dragon King Ghidorah to show up and challenge Earth's apex archetype-based predator Godzilla.