Which is better:
1) The bad novel you actually wrote, or
2) The idealized novel that you thought about a lot, talked to your friends about, planned out in detail, but never really got around to writing?
One more question: which of the two will teach you more?
Look, when you finally finish that manuscript, it will most likely suck. Be ready.
But at least it will exist.
Don't let the perfect be the enemy of the good. The act of completing something--even if it's terrible--will teach you a hell of a lot more than nattering endlessly about something you'll never write.
Isn't it hubris to hold back, never publish anything, and expect to improve as a writer?