For as long as we had non-sealed batteries I have always tossed them out regardless of their condition after 2 years or the first unexplained sign of weakness.
The thinking behind it is that the remaining life would be worth at best 15 or 20 bucks, and not worth to risk a breakdown over when on a trip.
The new AGM batteries are much better, but on newer bikes, with EFI in particular, the battery becomes a much more critical component, so I probably will not change this practice all this much.
Under the right combination of circumstances a defective battery can take out an electronic component if the bike is ridden any significant distance with it, and in the majority of situations you will not be able to pushstart an EFI bike with a dead battery.
No different then replacing worn tires when they are down to 1/8 thread depht as opposed to running them until they are completely bald.