Yes, your chances of success are reduced if you do not get funded by YC. If they're reduced enough for you to quit, you shouldn't be doing it anyways.

I define "success" as being able to quit my day job and work on the startup full time. Anything after that is gravy.

My chances of being able to do that if I'm accepted to YC: 100%

My chances of being able to do that if I'm not accepted to YC: 25%

The 25% is more than enough to keep me going and to keep my inspired. But I'd still have a better shot if I got accepted by YC.