Take, for example, the over-optimistic fiber build-out that
essentially terminated in 2000 - it's taken 16 years to use all that
up....
That sounds like it was in the right ballpark. Trenching is so expensive, only doing it every couple decades sounds like a reasonable plan (even better if you trench conduit that you can run replaceable cables in (which is what AT&T did when they took U-Verse into the Santa Cruz mountains).