Probably lowballed it to be safe. Wasn’t it and the original Zelda among the first of their games in the West to use battery backings?

Yeah, especially since the only gauge they had to make their estimate was probably how well the batteries performed in watches. Still, it's funny how they say 5 years (it even says that certain conditions may shorten that expectancy) yet some games are still just fine 35 years later.

Accidental longevity!

Better than modern technology's deliberate lack of longevity. I would not expect a modern device to still be functioning as well as my NES still does after the same number of years.

Sadly agree.

Of course, there is a bit of a double standard there, as the NES itself never functioned all that well to begin with. Especially since mine is the original model. So while I can say it's great that it still functions just as well as it always has, other people could be like, "Not that that was very impressive to begin with."

There are some that last the test of time, though we’ve had to replace many a battery over the years as it has been a little over 5 years since Zelda 2 at this point. Gameboy cartages seem to fail a bit faster though, but still around 10-15 years roughly before they go out.

If only replacing the batteries were a simpler task. But at the time, they probably expected people to either buy a new copy of the game when their save battery died, or have already moved on to the next big thing by time the batteries died. Especially as technology was improving with new console releases, game developers probably didn't expect people to still be playing NES games nearly 40 years later.