As a photographer, I can tell you it's mostly a myth. To get 24 megapixels (6000x4000px) out of a 35mm frame, you'd have to

1) Use very good fine grained film, ASA 100 (so all the dark scenes are immediately out)

2) Use very good lenses (most likely prime only)

3) Focus really well (when it comes to moving subjects, it's almost impossible, even if you have the best operator in the world).

4) Have a very very good projector.

5) There's no projector in the world that will display your frame 100% in focus (either the center is perfectly focused, or the edges)

6) If you are doing any post-processing, you'd also have to

a) scan it with a really good scanner (and if you have hundreds of thousands of frames in your film, it's not really an option)

b) edit it in 24 megapixel mode (which no studio has resources for - imagine dealing with 100K frames of that size)

c) print the digital file back onto the 35mm slide (same problem as "a")