Partly in response to @ChrisH, I thought I should check how much capacity a lithium ion battery potentially loses in prolonged storage. The answer is complicated. I think that potentially, lithium ion batteries can last several years in storage with only some loss of capacity. However, the answer is a bit more complicated than that.
Battery University has an unsourced table showing the (estimated?) capacity remaining in a lithium ion (cobalt chemistry) battery when stored at 40% capacity for one year. At 25 Celsius (77 F, around room temperature), it's 96%, and 98% if stored at 0 C (32 F). I don't know if the remaining capacity function is linear with respect to time or if it's bathtub shaped. Nevertheless, you'd probably want to arrange to store the battery at low temperature - above whatever the manufacturer specified minimum is, at any rate, but that minimum will probably be below freezing anyway. Storing the battery at full charge is harder on its life.
The article didn't explicitly talk about periodic recharging of a stored battery. However, all lithium ion batteries self-discharge, i.e. if not plugged in they still lose some charge over time. I think that if the battery completely discharges, that will kill it - the Battery University page recommends discarding the battery if the cells are under 2.00V for more than a week. I'd assume you need to periodically charge the battery, and to maintain optimal capacity that does mean partially charging it and then unplugging it. This site cites a recommendation from Panasonic to do exactly that.
If you are not able and willing to do that, then it may be better to just try to source a replacement when you need it or when you perceive that the company may be going under or may be discontinuing that model and its logistical support for the model. That's easier said than done, but so is periodically charging a stored battery. I can keep up with the batteries I have in service, but I don't think I would be able to maintain a stored battery like that. I suspect that the issue of future battery replacements is an industry-wide issue that hasn't been solved. You might be better off going with an established e-bike brand, since you could assume that they're more likely to support older products - but I don't know that this is necessarily true either.