fan theory that they're all dead?

No. This was made long before internet made up fan ideas wasted people's time.

Like this is a purgatory job and they're flying people back and forth from heaven but they're stuck in that god awful terminal and Lowell is an angel or something? Sure, why not.

Like Clarence in It's a Wonderful Life, you mean?

Well, that would be a plausible alternative interpretation of the series title. In order to either substantiate or disprove it, we need to pay careful attention each time a character (e.g., Lowell) leaves, and see whether or not a bell rings.

As for fan theories not being a "thing" until the internet -- well they were probably less prevalent, but fandoms and fan theories most certainly did exist. There were in fact fan theories even before television caught on, for stories appearing in print -- with one example of many being the theory proposed in 1943 by Dorothy Sayers to explain why John H. Watson's wife addresses him as "James" in one of Arthur Conan Doyle's Sherlock Holmes tales, namely that his middle initial stands for "Hamish," which is the Scottish equivalent of "James." Not a bad theory, really, since Doyle himself was Scottish by birth.

My fan theory is that they're a couple of guys who run a small airline.