I think it's people who knew the original Figment, who then share stories and videos of the original ride with others who keep the fan-base going and growing...plus the comic book series helped a ton. After everyone thought they were sold out, friend of mine actually managed to find one of the 35th anniversary Funko Pops in an odd place at Epcot for me. It was on a shelf behind a register (forget which store) with no other Funko Pops around.
ETA: And don't forget that at (I think) a D23 a few years back, Tony Baxter got in on the "bring back Dreamfinder" action and people went bananas.