That temperature is fine for chips. Worry when it gets to 80C+ (even then it'll probably be fine). Some cards idle at like 60+
Hardware will stop itself from overheating - either by drawing less power and slowing down / increasing fan speed - or turning off the computer alltogether.
Fatigue happens if the the hardware expands and contracts a lot (increase/decrease in temperature).
I imagine going between 40 - 70C wouldn't be much of a problem tbh, though I'm not an expert.
Temperature cycling is NOT fine for anything. It causes "fatigue" wear. Any 3rd year mechanical, civil or aerospace engineer or materials science major will know this without thinking 1 second about it. Since there are MANY MANY parts in a board and only one needs to fail to kill it, fatigue is a nightmare for them. Mining is not the norm for a gaming card ... gaming is. Gaming runs at whatever is required while you are playing and then goes back to typical idle when finished. This is cycling over long periods of time and much few times during the life of the card. With mining, it is happening as often as once per minute in this pool.
So, sorry, but you are absolutely incorrect about the damage temperature cycling can do, even if below 70C the entire time.