Stagnation is a normal thing and any system needs to be designed for a reasonable amount of it. Go on vacation for a couple of weeks and it will stagnate.
But this is all normal. A properly designed system will last for 30 + years, with glycol changes every 6-8 years. It is the improperly designed/installed/maintained ones that fail.
The biggest problem now is oversizing the panel area vs the tank volume and not sizing it to the load. With glycol, you really should not size the system to do more than 60% of the annual load. This means 2 - 4x8 panels at most. Vacuum tubes are worse because they get hotter at the wrong times and they fail way faster than flat panels do.
Still, dollar for dollar, solar thermal supplies more energy than a PV system does (although the gap is closing). Different technologies for different jobs, I say. Still, it is fun to experiment.
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