Regarding MTTF in electronic systems, numerous factors come into play. Thermal cycling can have an effect, but it depends on what the low and high temps are, what kind of parts we're talking about and how much they're derated, etc. Leaving your gear on will wear down certain types faster (class A power amps come to mind) due to current flow and hence electrochemical migration. So depending on what kind of gear you have, how it's designed, and how you use it, turning it on/off vs leaving it on is truly component specific; making a blanket recommendation is not possible.
FWIW - I work as a reliability engineer in the semiconductor industry.
FWIW - I work as a reliability engineer in the semiconductor industry.