Phones aren't unibody, in that they always have a front and back. The front may be the screen itself, but it still exists. (There would be no way to get the electronics and battery inside if it were not so.)
The iPhone 4 was probably the easiest iPhone to disassemble, and to compensate for that, Apple used proprietary pentalobe screws to deter the casual user. Still, with a pentalobe screwdriver and a suction cup, it was trivial to open the phone. The connections inside, however, required a degree of dexterity to carefully remove and reattach. Could those connections be made easier? Probably. So Apple could have made the phone easier to service in at least one way, and probably two, and at least one of those obstacles was deliberate.
Compare that to something like the newer iPads, which require the application of heat to soften the glue holding the screen in place. There's nothing user-friendly about that design. There's something to be said for the aesthetics of a sealed case, but it would have been trivial to use screws through the back instead, as with MacBook Pro's, and that would make the devices far more user-serviceable.
Clearly making these devices accessible to the user would be beneficial from a cost standpoint, but it would be beneficial from an environmental standpoint as well. The most environmental choice is almost always to continue using an existing product instead of using the resources to produce a new one, and this is as true for electronics as it is for cars. How many more people would replace their batteries, or cracked screens, for $5 or $10 instead of buying a new phone? Many of my non-tech friends are still using their 3-4 year-old phones, or 10 year-old iPods, and I suspect they will continue using them until the devices die, as much as Apple or Samsung would like them to buy a new one every two years.
Personally, as long as I can get a device open without breaking it, I will always service it myself. That said, I'm getting ready to upgrade my iPhone this week, as I do every two years. And like many tech geeks, I make up for it by rarely buying new clothes. ;)