Batteries: Whether you use your bike all the time like Brambor (kudos!) or just the ocassional weekend, your battery will degrade and lose capacity, even if you maintain it per the manual. Would expect no more than 4 years life before the range becomes an issue.
I wouldn't buy a second battery until the first one wears out.. Youre just wasting money, unless you need the extra range. I bought two batteries with my first bike, and the backup battery degraded just as fast as the main battery, even though I hardly used it.
Motor: Direct Drive Hub motors can last tens of thousands of miles.. Motor that have integrated controllers will have a lot more problems, so it is better to buy a bike where the controller is not part of the hub motor. can't speak about the other types of motors.
Everything else will age just like the rest of your bike.. Electrical connections can be troublesome in cold and wet climates, so it just comes down to a quality build and your location.
My personal experience was that every major component failed numerous times on my expensive first eBike over 5 years, except the main battery. My second eBike, also expensive, has been trouble free for 2500 miles. I think these manufacturers are learning from each other.
I plan on selling my current eBike after 3 years, and buy a better one!