That's not how engineering works. There are good designs that can't be achieved while making things replaceable. For example, the reason screen glass often isn't replaceable is because it's optically bonded to the underlying LCD, which improves sharpness, reduces glare, and reduces parallax when using a pen. The reason batteries usually can't be replaced is that it saves space and weight to use naked Li-Poly battery packs molded to the available space. Making say a back case removal reduces structural rigidity and reduces space available for a battery. That's engineering--making trade-offs between features people may not care about in favor of features they care more about.
Most phone batteries are electrically compatible, just not physically compatible. I have used Droid phone batteries to power HTC phones by holding the battery in place against the contacts, even though it didn't fit into the phone at all.
On the engineering side, we should push for standards, and for components which operate the same at higher and lower volatges (2.0v, 2.2v, 3.0v, 3.5v, 5.0v, etc) where possible.
There are standards, but 3.5v isn't one. Big waste of power though to run a 1.2V part at 5.0V, and transistor geometry to make it capable of that. Most FPGA and some micros have separate rails for I/O and a low voltage core, like 1.2V.
Most people could stand slightly thicker phones. We are dumping huge amounts of toxic stuff because phones are designed for planned obsolescence. If there were pressure to use replaceable or upgradeable parts then we would see thinner phones with these traits, but without that pressure we are going to get more planned obsolescence. That's engineering too.
Sure in some cases engineering is the reason, but then some manufacturers put pentalobe screws on their phones to make things as difficult as possible for the consumer to look inside the products that they paid for.
