Sealed Battery = Planned Obsolescence
I know there are a lot of people that say having a sealed battery allows a device manufacturer to make the best use of a phone’s internal space, allowing you to store more battery capacity in a smaller chassis. I am not saying that this isn't true, but I think it has more to do with planned obsolescence that it does design aesthetic. Modern day smartphones are powerful enough that the improvement to specifications are becoming increasingly more miniscule than ever before. Those devices will stop getting the latest versions of their respective software, but the features that these recent versions have brought haven’t been anything noteworthy to the average consumer. I think if you handed a normal person a device running Android 4.1 and a device running 4.4.2 that person wouldn't notice a lot of change that affects day to day usage.
So what is going to drive normal people to upgrade a perfectly working device with a multicore processor, great camera, and the ability to download the same apps as the new phones can? Its probably because after awhile they can no longer make it a full day with their smartphone. This upcoming bit is anecdotal, but I think that it is noteworthy since I used to work for a wireless carrier so my sample size is relatively large. I see a lot of people still holding on to the Galaxy S2 because it has all the meaningful features they are looking for, they can download the same apps to their device that the Galaxy S4 and S5 can, and when the battery life started to get unbearable, they bought a new battery and called it a day. In that same vain of anecdotal, I see people upgrading from the iPhone 4 strictly for the fact that the phone can no longer last a full day on a charge.
Overall, I don’t feel normal people care too much that a phone can be 8.5mm thin with a sealed battery versus 10.5 mm "thick" by having a removable one, because those same people are going to put a gigantic case on their phone anyway. It is getting more difficult to create reasons why someone would need the next incremental update to their current device, but when they can no longer last a day because of crappy battery life, now you have your reason.