People need to understand how electronics actually fail. Electronics doesn't experience 'wear' like mechanical systems (like car engines) do, so running electronics 24/7 is rarely detrimental if kept within its design envelope (temperature, voltage, humidity, etc).
Solder-related failures (often the dominant type in mass produced electronics) are driven by thermal cycles (heat up/cool down), which mining cards don't experience. The main failures that are driven by operating time are mechanical parts (ie fans) which can easily be repaired and also depend heavily on how dusty the environment the fan operated in.
Electro-migration, whilst loosely linked to operating time, is far more affected by operating temperature and voltage. If kept within its temperature and voltage limits, electro-migration is likely to be so slow that something else (ie solder or fans) will fail first. Electro-migration also doesn't tend to insta-kill electronics, instead it is likely to result in gradually higher leakage current (so higher idle power draw and temp) and worsening OC potential. As mining cards are usually undervolted and underclocked to maximise power efficiency, electro-migration is likely to be less severe in a mining card running 24/7 than a gamers video card overclocked and overvolted running for 3-4 hours a day.
Lastly, most miners keep their rigs pretty clean and run in open systems/racking, compared to many gaming systems that are under desks collecting a lot of dust. I'd actually trust an ex-miner card over a Jo Bloggs gamer card, unless said Jo Bloggs could show how well they maintained the card (dust free, not overclocked, etc).