As far as I can tell, the problem is the manufacturers stop making the batteries for particular phones after quite a short time. So by the time you need a replacement, you're in the territory of "reconditioned" batteries (in the best case).

I suppose I could have the foresight to buy two batteries when I get a new phone, although I've never done that and I'd have to work out how to store the second battery so it wouldn't go dead or get lost, and it increases the initial cost of the phone even further.