Heyy, there's proof that they shot everything Fede scripted! There's the original ending (And the old guy in the car was originally supposed to be Bruce but he refused to do it), there's the burning cabin scene with the Deadite eyes and join-us-ing, there's the "We're gonna get you" clip, and there's the chainsaw bit.
To be honest, I don't know why they lost both David scenes. I think Shiloh's generally a classic of non-acting and his expression sort of ruins the Deadite eye reveal moment but both edits are much more interesting than what we got. The Mia/Natalie scene likely got axed because the GIVE ME BACK MY HAND moment on top of We're gonna Get You... comes off as unintentionally ridiculous and overly referential, which is a shame because Jane is killing it in that moment. I don't have any idea why they didn't weld the alternate ending onto the main body of the film if the scene was excised for Bruce's cameo.
The movie was too streamlined as it is, but it could've used this extra meat - and if they'd rather streamlined over coherent, why the hell didn't the axe all the Grandpa stuff and put this in instead?
In any event, I think Nick el Ass is right - the movie had a vogue but it never developed much of a devoted hardcore fanbase; people liked it generally but it didn't leave much of a cultural impact.