The problem with removable batteries is that you sacrifice water-proofness. That's why iPhone batteries aren't removable: Apple found that, on average, people prefer a water-proof phone over a replaceable battery. If you make the battery to removable, you can't use sealants anymore, you're just relying on a gasket.
Personally, I would rather have a phone that can handle being dropped in a sink full of dishes and am happy to pay $80 every 3 years to have the battery swapped.
As others have mentioned, the Galaxy S5 was water resistant and had a replaceable battery. Phones are generally not water proof, so don't go dunking them in water expecting them to work. Use a case for that, also if it floats all the better.
Adding a sealant when you close it isn't a huge chore, although it's nice to be able to just replace the battery without any tools, it's better than charging. Also it would still be water resistant until the first battery change.
Watches have replaceable batteries while having replaceable batteries, too.
As a side note, if you drop electronics in water, take the battery out before taking them out of the water, so that the short doesn't destroy them. The battery will be toast, but that's cheaper to replace.
Galaxy s5, all of the galaxy xcover phones. And also user removable in a sense that a user must be able to remove it with widely available tools or the tool must be provided. So manufacturers will probably start using screws and give you the screwdriver or something. Probably would also mean longer software upgrades since users probably would not feel the need to buy a new phone every time the battery dies.