I took a 1/2" air impact wrench (250 ftlbs but it was a gift so..) and could not get my anode loose. 3/4" is up next I guess. Anyway, back in 2000 I bought and installed 2 US Craftmaster 40 gallon electrics and plumbed them in so that when one failed (we have A LOT of lime) I could throw a couple valves and have us back up and running in a few minutes but get to the heater repair when it was more convenient for me.
These are 6 year warranty machines. About every 2 years the bottom element will go out because the whole bottom fills with lime. So they get cleaned out and reset on a regular basis. I think the idea of regular service has extended their cycle, given that they are 18 years old but have roughly 9 running years each. And one other detail, lime is Alkaline so perhaps easier on metal than a lower ph hot water.
I would love to get the anodes out, but I bet it will take more than 500 ft lbs and somewhere up there the tank is liable to fail. Maybe I should just run them until they leak.
Gas is incredibly corrosive on heat exchangers but it seems like that would be incorporated into the heater design. To reduce corrosion or extend life I guess you reduce the flame? But even on a slow boil ultimately the same work gets done so I do not know.