I agree completely with what you said and have always been mystified with this concept of “necroposting” as well. Of course one should not bump an ancient thread about a solved problem with an unhelpful reply (such as “bump”), but this is not a problem with necroposting per-se, just with low-quality contributions that revive old threads and clutter the top of the list with misleadingly active discussions. Unfortunately, I think the concept of “necroposting” resulted from a hasty generalization of this behavior onto all thread revivals, whose enforcement solidified its place in netiquette ever since.
With that said, there remains some situations in which the appropriate behavior is not entirely clear. For example, let’s say that there is an old thread whose last post was a month ago or longer, and the discussion in that thread resolved into a decision that concluded it quite nicely. But if one was not party to this discussion and wishes to challenge the decision or otherwise reopen the discussion in the hopes of reaching a different conclusion, or perhaps just append their own observations after the fact, then how should one proceed? Reply to the old thread with the intent of disputing its conclusion, or create a new thread that attempts to rehash it?
More generally, at what point is a thread too “stale” or “complete” to be revived, as opposed to rediscussed in a new thread? Your logic would suggest that one should reply to the old thread in all such situations for the efficient reasons you specified, but the optics of unsettling a settled discussion and disturbing its pristine resolution with a belated reply weeks or months later remains unappealing. In this sense, the concern I have remains in the realm of propriety rather than reason: doing so may be logical, for a certain conception of logic, but is it appropriate?
That is the crux of my concern: the best way to disturb the peace.