I have the same doubts as you do, but as an electrician that does tons of
work with plumbers, I can tell you that they all love the stuff, they
swear by it, and assure me that it won't fall apart ten years from now.
I'm still a skeptic, but for whatever reason, the stuff has made believers
out of plumbers.

Well, the "failure mechnism" of an o-ring seal is wear of the o-ring with
pressure fluxuations. But in a domestic water situation, the temperatures
do change much and the pressures aren't extreme. That minimizes wear and
fatigue.

IOW: it's a near static situation.

The "record" in static situation for O-rings it quite outstanding: WWII
aircraft found in deserts after 50+ years still had charged accumulators
(nitrogen over hydrollic oil).

Over the past decades plumbers have had to replace gross amounts of plactic
plumbing from problems that showed up after 5+ years. Compared to that, I
would expect the o-rings to outlast any plastic pipe.