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It's a Wonderful Life was an obscure cinephile favorite
until some time in the late 70s or 80s. It was made independently (at least in a sense comparable to most modern movies labeled "independent") back in an era when that wasn't done, did poorly at the box office, and then fell into relative obscurity despite its big name director and cast. It eventually slipped into and back out of public domain (it was reclaimed despite a lack of renewed copyright on some technicality) but I think it was the fact that for a while it could be shown on TV for no royalties that eventually gave it the exposure to be widely recognized as a classic. So if "cult" doesn't mean that it has to be some kind of genre film or something outre then its popularity was limited to what is more traditionally called a "cult" fanbase for a pretty long time, longer than Blade Runner or Spinal Tap and certainly much longer than The Big Labowski.
But I agree with everything else you said.
Here's what Rolling Stone readers think are cult movies: https://www.rollingstone.com/movies/pictures/readers-poll-the-25-best-cult-movies-of-all-time-20140507
I think including The Blues Brothers in funny since that movie transformed Chicago's dying blues club scene into a thriving manufactured-for-tourists scene. Plus it was the 10th highest grossing movie of 1980. It's the ultimate Fake Cult Movie. I like it though.