My take on it is just a matter of priorities. But before that list, it's also important to note that the vast majority of consumers will have no idea if their battery is removable or not and won't do hot swaps in their day to day lives.
First, I believe devices with poor battery life shouldn't be released. If the battery life is terrible and letting you swap it out is the fix to that, that's a bad device. Second, making a battery removable means putting limitations on the size, shape and location of the battery that aren't necessary in any other circumstance. Third, making a battery removable, until this design, meant a devastating compromise to the build quality (and usually a compromise on build materials) and adds in a host of tiny, breakable plastic parts. Finally, the presence of a removable battery, for those who know it is there, promotes an unhealthy (for the device) set of battery habits that include letting batteries drain to 5% or 0% and then swapping, doing battery pulls to reset and purchasing knock-off third party batteries.
In the first issue, having a phone designed with better battery life is better than having to worry about battery life to the point that you feel you need two batteries. On that note I'd add that battery innovation should take place in the form of getting us batteries that last for "weeks" and aren't measured in 1-2 days or even hours. On the second point, it should be obvious that having a huge 3000 mAh non removable battery that is placed in a more intelligent position for heat management would be better than having a good sized 2500 mAh battery that is removable. (LG usually still goes for thicker, higher capacity devices but the point is that the size and efficiency could both be better if pulling it out of the device wasn't a consideration). This carries in to the third point, that taking that 2500 mAh battery out repeatedly is putting unneeded wear and tear on an already compromised design. And the final point - people are slowly starting to learn about battery temperature regulation and charge cycle optimization, but this is a long battle. The vast majority of people still have no idea what those things are and so real solutions to those issues need to take place on the OEM's side, not on ours.
All of that said, if there is going to be a removable battery, so far I like the idea presented here much more than a removable back. This appears to have less in the way of build compromises and could lead to a device that is designed much better than what the typical device with a removable back can be.