For the last friggin' time: it has nothing to do with believability, it has to do with tone. Ghosts, magic and fantasy vs. aliens and science fiction.
HOW IS AN INTERDIMENSIONAL SPACE BEING LESS VIABLE THAN A BOX THAT IS LIKELY INTERDIMENSIONAL AS WELL?!
It's not about what's more viable or believable. It's about how the dark supernatural stuff of the original movie was a MUCH more natural fit with Indy's world-- seeing as much of what archaeologists tend to dig up has a basis in ancient religion.
Archaeologists aren't digging up a whole lot about ancient aliens and UFOs. The only research out there comes from the ridiculous fringe field of "UFOlgy," which no legitimate scientist takes seriously. The Mayan/alien is just a bunch of new-age nonsense that originated in the 20th century and belongs more to the world of supermarket tabloids.
You might as well have Indy face Bigfoot or "Batboy" next, if that's the standard we're going to use now. lol