If you even look at a photo of a new fiber gear it is clear that the teeth
are machined. But it is a LOT cheaper and easier to machine than steel.
Also if the little nozzle gets plugged, the steel gears will go to pot pretty
quick and VERY noisily- the fiber gear will last a while because it has
oil soaked into its surface.
Note that there was a post not long ago about steel gears being noisy and
then the poster pulled the timing cover and discovered his oiler nozzle
was not open. Fixed that and the noise was VERY hard to discern.

With stock cam and valvegear I think you would have to drive VERY hard to
make a significant reduction in gear life. I am sure that there are age-
and temp- and moisture-related factors that work together with manufacturing
variations so that the life is pretty unpredictable with an old gear.

For example suppose you took a fiber gear in pretty good shape and cleaned
it very well, getting all the superficial oil off of it and out of the
surface, then stored it in a damp garage for 5 years, with winter freezing
and thawing.
That gear would not last nearly as long as one that was removed, not cleaned
and stored in a box in a warm dry place.
--
George Downs Bartlesville, Heart of the USA!