That is, it isn’t something like a Krell machine.
I watched City on the Edge of Forever for the umpeenth time the other night, and had a flash of revelation.
I suspect we all think that the Guardian was the centerpiece of the once advanced civilization that was on its planet. That it was the height of advancement. That it survives, like the Krell machinery, because it was built robust. I mean, why not? The ability to travel through time is not only a great scientific advancement, but a source of immense power.
My “flash of insight” is that, instead, the Guardian is a kiosk from some long-destroyed shopping mall. Consider how it works - it shows historical periods as a series of speedy images. It can’t be used for precise travel to time or space. (“I was made to offer the past in this manner. I cannot change.”) Even in Kirk’s time, time travel is more precise than that. Who would create such a useless machine?
Some cheap novelty machine manufacturer, that’s who. Assume instead that the Guardian is just a toy. Put a quarter in and make a wish. But instead of your fortune, the machine can send you back in time. And when you’re time is up, it returns you. Obviously, the machine yanked Kirk, Spock and McCoy back, they didn’t initiate travel at their end.
The machine is a giant reality-based role playing game. Go back in time, change something, and watch the results. That would explain why the landing party wasn’t immediatly poofed out of existance when the time stream changed. Your friends with you get to see the changes, then they try their own changes. Endless hours of enjoyment! See how much YOU can alter the present! Only a quarter!
When at the end, the Guardian says “Many such journeys are possible. Let me be your gateway.” it isn’t some lonely sentient machine, or a friendly alien offering to help. That’s its sales pitch!
Because of the way the Guardian was made, it will be sitting there until the end of the universe. Worlds may change, galaxies disintegrate, but a the Guardian always remains.