That very much sounds like, for some reason Google has gone from viewing that particular page template as "decent, worthy of rankings" to "ok, will rank if I can't find something better". One thing I am wondering, if you have been hit by this: https://moz.com/blog/google-review-stars-drop-by-14-percent
... which is also related to this:
https://webmasters.googleblog.com/2019/09/making-review-rich-results-more-helpful.html
Specifically where they say "Self-serving reviews aren't allowed for LocalBusiness and Organization. Reviews that can be perceived as “self-serving” aren't in the best interest of users. We call reviews “self-serving” when a review about entity A is placed on the website of entity A - either directly in their markup or via an embedded 3rd party widget. That’s why, with this change, we’re not going to display review rich results anymore for the schema types LocalBusiness and Organization"
It seems as if 'something' existed on your product detail pages which Google was valuing highly, which they no longer value at all. Thus you aren't seeing complete drop-off, but a high correlation between declining (or removed) results and pages utilising that feature
Basically self-hosted reviews and some embedded reviews 'no longer count' towards Google rankings (at all). The news broke in September 2019, but I wouldn't be surprised if the roll-out was more recent. Moz posted that they noticed movements on Sept 24, which is very nearly November. As we know, these types of updates tend to slowly crawl across Google's query-spaces, it's not often true that everyone gets hit at once
Maybe your site is just in the late batch
I especially agree with your first point, as I believe that infrastructure (structured data) will be critical to the 'voice-web' which may replace large swathes of 'visual' search. Not all of it, but large chunks of it