No film is talked about as much as when it first comes out, with a few exceptions. Geddit?
Gone With the wind, Casablanca to name a few (so many out there that it is more than than a few exceptions), so no, I don't understand what you say
I already worked out that you didn't understand (see above).
As to the OP's choice; I'm not sure that a low budget, B movie which does much better than anyone anticipated can be called underrated. I'm thinking of Pitch Black, specifically. Coincidentally low budget B movie can also be applied to Casablanca, although it was a much more spectacular success than PB, obviously.