Just one?

I s'pose everything's relative, but my take is yes, the 'bar may be #1 while what separates it from #'s 2 ~ 12 is what the final costs could run to return your body to something resembling what it was before part #n broke, contributing to your falling whilst riding.

Steering? Yeah, you need to be able to steer.

Brakes? Some say 'Why brakes? I want to go fast!' until they don't.

Seat post? Unless you mash 100% of the time, it's nice to have someplace to sit while you rest your legs.

Then there's them rims, with the tires with which you stay in contact with Mother Erde whilst riding (hopefully) firmly attached.

Bikes are machines after all. Failure of one part will affect the performance of all the other parts.

CF is great under tension – as long as the epoxy encapsulating it is intact – 'cause it's not so good in compression by itself... why the epoxy is there.