Screen glue is necessary for waterproofing but I agree with what you’re saying.
- Yes in the sense that the glass needs to be glued (or rather taped as a double-sided tape is used) to create a waterproof seal as the small size of these devices does not allow for a more traditional clamp fit using a rubber gasket.
- No because it does not have to be the main body of the device to which the glass is glued/taped. I use the Motorola Defy+ [1], an 8 year old water/dust/shockproof Android device which shows a phone can be both waterproof as well as repair-friendly: the glass is glued to a frame which in turn is attached to the phone using screws. Disassembling the device is easy and quick, all it takes is a small Torx driver and a guitar pick. Assembling it is just as easy, the result will still be waterproof as long as all the rubber gaskets are fitted. I have 5 of these devices in use for different purposes ranging from 'dangerous work phone' to trailer camera and remote-controlled media player (using MPD for Android).
In short, it is possible to create waterproof devices which can be repaired without needing to break one-time seals.
I think it would be tragic if the iPhone looked like those Motorola devices. That thing looks hideous. This is strictly against Apple's design philosophy, and for good reason.
(* Technically I guess it's the iPhone 4 that came out same year as Defy, but arguably close enough)
I always thought modern smart phones should have started as a black slab like the 2001 monoliths and just stopped there.