There's a mixture of degradation mechanisms at work. Some of them, like light induced degradation to the actual cells, can reduce output but can't really destroy the system. Other mechanisms really do "break" the module, e.g. backsheet delamination, accidental or malicious breaking of cover glass, or extended partial shading that leads to thermal runaway that can melt solder and/or put holes in the backsheet. As soldered connections age, weaken, and fail they can lead to thermal runaways that destroy neighboring cells too.

I believe that there are already some panels in the field that can last 50 years, and that it will become more common in the future, but 20-30 years is the safe, evidence-backed estimate for now.