I'm of the belief that flushes can be bad on an old high mileage trans, but a fluid and filter change never hurt anything IMO. If it's dirty, change it.
Correct, the back fllush, total fluid exchange machine is death for any transmission, but fluid and filter is what it's intended to have done. No engineer ever sat around going, "well if the clutch material stays in the fluid, its the same as having it on the clutch, so they better not change it..."
However, if you were to unhook the lines at the radiator, and let it run, pumping all the fluid out at it's natural rate, it wouldn't hurt a thing.
There's a few thing's you can do to any auto to prolong it's life, cooler, firm up the shifts, try not to overstress it with large tires and tall gears, or towing in overdrive up grades where it hunts through gears, of course, fluid and filter changes. In the end, it will wear out and fail, there's no way around that. And I suppose, if it's a 500 dollar beater you just need to run to town in during the winter, then why waste the money, but if you're counting on the thing, do all the maintenance you should be doing.