I personally prefer non-removable batteries and there is a number of reasons for that preference.
The concept of non-removable batteries was introduced by Apple in their iPhone series, and that was in 2007, where there was not a single phone which came with a non-removable battery, so iPhones for sure was faced with a lot of criticism, but now looking back, we know Steve Jobs did that for a reason.
So here are some views why every single smartphone out there has non-removable batteries:
1. Water-Resistance: To make a phone water-resistant, there should be less number of openings and better internal sealings from outside to prevent the water from getting inside the chip and frying it. So if there is a cover for your battery that is easily removable which could allow water to seep in, then that is a bad idea for water-resistance. I hope you get the idea.
2. More space for other components: Having a removable battery requires an extra layer of plastic(or metal) to protect the internals from everyday impacts as the external is open to the surroundings. This extra layer can add to the thickness, while smartphone companies are racing to make their phones slimmer.
There are some other reasons too like batteries which are weirdly shaped that are designed to pack more power which makes them harder to remove if it is in a battery removable smartphone(for example, the iPhone X uses an L-shape battery consisting of 2 cells). Nowadays all phones have to be stylish so that it is attractive to the consumers, having a metal or glass back plays an important role in premium design. Metal and glass bodies do not go well with removable batteries.
So it is pretty understandable why every single smartphone companies use non-removable batteries in their smartphones.
Deepak Mathews Koshy