Where to begin?
Evacuated solar tubes come in two different types (or they did ten years ago).
‘Heat pipes’ which heat the antifreeze glycol indirectly or ‘direct heat’ systems which heat the solution directly.
As long as the original installation was done to a high standard, there should be no reason why a solar tube system shouldn’t last for thirty years if looked after properly.
If an individual tube fails, shouldn’t mean the whole system fails. You’ll get a slight loss in efficiency, but as these systems can reach between 40-70 degrees Celcius, then a slight loss in performance won’t matter.
Try flushing and recharging the system with fresh glycol and/or check/replace the pump and management system. It could just be a case of a cheap fix that may give you decades of free hot water.
Hope this helps.