It'll be way too expensive to have a build your own phone. And it's not what the majority of consumers want. Apple has proven this time and time again. An unserviceable phone, an unserviceable tablet, and now unserviceable laptops. None of which have the simplest of battery swaps available (hell even the ram is soldered on board now in the MBPs). They've built the largest computer (mostly mobile) empire on hardware that is idiot proof and has no options. This is what consumers want. A build your own phone would be fun, but just not practical and way too expensive.
I'll get slammed, I know. Fanbois unite and all that. Oddly enough I'm typing this on a 5yr old MBP that I will lament when it finally goes. I don't like ios and most likely will never upgrade since I do not like their new models. I know this one has been through a lot with me and performed flawlessly. I've done some hardware hacks and it's been fun.