I tis going to need at least three or four battery sets over 20 years as no current battery has an indefinite shelf life once installed, even if not ridden.
Maybe, or maybe not. We shouldn't be talking about shelf life, which means no recharging, and will kill from self discharge. Anything suffers from neglect. Similarly if we are recharging every day, or two or three times a day, lifetime will again be relatively short. But a usage scenario that draws the battery down to 30% and charges to 80% could last much longer. Folks who ride 50 miles a week, at low average speeds, (like my commuting scenario), might only need to recharge 60 times a year. Depending on the temperature of my garage, that battery might last an impressively long time. Probably not the 1000 recharge cycles / 60 recharges/ year = 16 years, but perhaps.
Industry standards say that batteries are officially dead when they can only be charged to 80% of their rated capacity. I bet there are a heck of a lot of gas powered motos out on the streets that, due to voluntary modifications, age and/or neglect, can't achieve 80% of factory claimed fuel tank range. We just fill them up more often. This is how a lot of us treat our cell phones, and laptops, they don't get new battery, they spend more time being charged
At least one person, much more educated than me, thinks that we can expect much more life from our LI-ion batteries, up to 20 years:
https://insideevs.com/news/317649/expert-what-you-know-about-lithium-batteries-is-wrong-can-last-up-to-20-years/I've got an old sealed lead acid car battery in utility service, that if it isn't already that age, is approaching 20. I'm sensitive to its needs and it does what I ask it to do, standing in for moto batteries on bikes being serviced, jumping cars, running an inverter, whatever needs doing. It gets re-charged when it needs it, a period that can be measured in weeks or months.