"There are many reasons the battery is not easy replace. Creating a user serviceable battery compartment wastes valuable space inside the unit. The battery itself is usually a custom shape for greater efficiency. And warranties are harder to accidentally void by not being able to easily swap the battery with a non-standard replacement"
All extremely lame (and should be illegal) excuses.
Space should be a secondary concern to safety. If you must be bigger, so be it. Just tell the customer the law (fire safety regulations) doesn't allow you to be too thin, just as you can't buy a new car without seatbelts anymore. Ignoring safety considerations can be considered Darwin Award candidacy.
As for custom shapes, that's just cramming combined with Planned Obsolescence.
As for voiding the warranty, you can't fix Stupid. Those who care will take care no matter what it takes. Those who don't would probably find a way to run it over regardless.