It's pretty tricky though, and even for a tinkerer it's not trivial. I changed screens, home buttons etc, and find that much easier than changing batteries. I haven't failed to replace a screen on neither the 5 or the 6, but I failed with a battery on the 6, due to broken battery tab and not enough guts to heat/bend it out (I left it for someone else).

Now, I completely agree with the design decisions of the iPhone. The completely sealed body is great for water resistance and having the battery replaced is still pretty cheap, especially if you use (I usually just let someone replace with a 3rd party battery for cheap, as this usually takes place after warranty expired).

The battery needs to have extra space (or bad things will happen) so it effectively has to be fixed on one side inside the body. Gluing it is a pretty cheap solution to that problem.