> If we only traced young objects

Careful; just because you only collect in the new generation doesn't mean that you only trace the new generation. From my understanding a rudimentary generational GC design is to include the old generation in the GC roots so you don't need to examine everything in the new generation - if it's not reachable from the roots it's implicitly dead, and the latter category is assumed to apply to most objects in the new generation under the generational hypothesis.