Cynthia does this thing where she will write an entire first draft of a novel manuscript and then throw it away. She'll then do a new draft, which will be her true "first" draft. (When she talks about this to writers' groups, she gets audible gasps of horror).
I don't tend to do this, but I do think that writers should be prepared to perform major surgery on manuscripts, i.e., not fall so in love with what they've written that they fail to recognize when major portions aren't working and likely will never work. I did something similar with TOFU AND T.REX, but not intentionally. I'd sent my editor a manuscript I wasn't happy with, but was not in a place where I could see (or was willing to see)what needed to be done. My editor at the time sent me a letter which basically identified in a coherent manner the things I didn't like and thought weren't working. After editing, the only thing that remained from the first version I sent my editor to the eventual published version was the interview scene at the Brandenburgs. About ten pages. Note that the version I'd sent my editor was far more polished than your usual "first draft."
At the early stages of drafting, I do have a "viability threshold." Basically, I find it very easy to write the first thirty pages of a novel. I have several manuscripts based on great ideas that are about 30-35 pages long. Great introductions, but the stories never went anywhere, or just went badly. Usually, the problem is the plot, but sometimes it's the characters. In the earliest version of the manuscript that yielded something that went into NINJAS, PIRANHAS, AND GALILEO, there was a character who was so annoying that I had him throw himself in front of a train. A moving train. It didn't miss.
Anyway, I'm very happy that my first (approximate) draft of my new WIP, RM, came in at about 110 pages. It's not the complete story, but enough that I know what's likely to work and what's not. Also, unlike in TOFU, I'm keeping several of the scenes; I am utterly deleting two major sub-plots, though. They just didn't pass the "cringe-test" (Things fail the test if, after letting the manuscript sit for a couple weeks, they cause me to literally cringe or recoil upon re-reading.).
Looking forward to this weekend and drafting new scenes...