We'll see how it turns out in 15 years. They will likely drive 50k in that time frame and park in moderate climate under shade, and by that time FIL will be 88 and MIL will be 85. By 15 years even if the N-1 cells are not bad, they'll likely be heavily aged.
Again, 15 years is a long time, at least 10 of it will be under the battery warranty.
Based on their Camry's condition, I'd say by then the struts will be shot, bumpers rattle can spray painted, sun visor ripped (he was pretty harsh on sun visors), interior stained, buttons and knobs cracked and letters worn off (they put a lot of lotion on before driving, and oil is bad for plastic), some leaks and squeaks here and there, the usual aged stuff.