From what I remember of the Eternals weren't the Celestials pretty much based on the Greek Titans - Kronos and Uranus?
Only in the sense that there is some powerful race who came before. In Greek mythology, the titans are supplanted by their children. I think only one Celestial has been slain in human history.
In the Marvel Universe, the Celestials are still around, just not here right now.
The "gods" being an extradimensional race works for me, whereas the Earths superheroes are alien science experiments doesn't.
The Celestials (in the comic books) are not technically extra-dimensional (like, say Dormammu is). They are among the first living beings in the universe.
In the comic books, Asgard and Olympus are extra-dimensional. In the MCU, it may be they are simply other planets, far away.