I find bike battery life amazing. I too have a 98 Triumph still with its original battery, and a 95 Kawasaki farm quad with its original plugs and battery ! despite daily use, or should I say because of daily use !
I find car batteries don't last anywhere near as long, had one go after two years but in general if I get 3-4 years out of them I think I am doing well.
In this part of NZ (northern) we have quite a temperate climate so no heavy cold starting which I would imagine puts a severe strain on batteries.
On another long life, I have a 97 John Deere tractor with the same battery from new !
Now having said all this watch them crash out bigtime.
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Beats kick-starts !