At that two year point you get the choice of a ludicrously marked up OEM battery. Something like $70 that could have gone towards a new phone. Not to mention it might be a fake or degraded from sitting on a shelf for those two years. Or you get an amazon china special no brand and hope it detects, has okay run-time, and doesn't melt down or explode.
I guess people like those options...
Personally I prefer that option buuut, this brings up the point about my L830. Tried looking for a spare 6-7 months ago and recall availability was pretty much non-existent here in Canada, did the same a few days back and to my surprise still a rare commodity. As you mentioned, the few I did find are overpriced and the chances of getting a "used" one appears very high, even found listings of people selling their used ones for ridiculously high prices. Think in my case the appeal for the phone or lack thereof is playing a big role here, had no issues finding same for my Samsung Ativ S when it neared the 2 year mark and prices were more than reasonable.
In the end it comes down to personal preference obviously but do think it is a good thing, at least the option is there if the need arises whereas when built-in, most would need to send phone to a repair facility and doubt many would bother. Too many devices in general today are becoming designated as "throwaways" which isn't necessarily a good thing, it may have outlived "ones" expectations yet I'm sure many others would welcome it with open arms, if it can't be readily/cheaply revived then it does no one any good.
Besides, in my opinion the trends of deploying non-removable batteries, lack of expandable memory etc. is just a marketing scheme to keep a stranglehold on the consumer.