I wonder how many of those batteries are truly dead when they get replaced. Last winter I drained one battery; this winter it got drained again. But it's limping along after being charged up. Another battery took a deep discharge also. Both are probably end of life, but a bit of charging and they still are pulling their weight, even in the middle of their worst trials.
I don't blame those who want to proactively replace--but I wonder if they are a bit too quick to replace? One good charge and maybe another year's use?