This is what's going on in my brain, then. Welcome to the crazy.
Brain: Work on Post-Apoc Ninjas. It's pretty good, and it has a lot of the same feel as Air Pirates.
Me: But I'd have to rewrite over half of it. It's so much work! Can't I just work on this New Shiny over here?
You'll have to rewrite that draft, too. It's less work to revise Ninjas.
But what if it's not? What if I write something that's mostly good on the first try?

. . . But if I rip out half of Ninjas, it'll feel like I wrote it for nothing.
Look at it this way: you still have half of a good novel.
What if I rip out more than half?
It's still part of a good novel. It's more than you had before you wrote it.
But what if I revise Ninjas and it's still not good? All that work will be wasted!
That's what you say before you start every draft.
I'd have to revise it AGAIN!
Look, what's it worth to you to write a good novel?
I HAVE written a good novel. It's called Air Pirates. Have you read it?
And how many times did you revise that?
. . . I hate you.
So long as you finish something.