As far as replacing parts is concerned, you can always pop a new battery or memory card (at least on non-iOS devices). By the time a non-replaceable part is gone, chances are that other parts are also getting old and you probably need a new phone anyway. If you want to spare the money buy a ~$400+ phone, and if you don't, you can get a ~$100 model that does everything your old phone did, and use the old phone for a simpler purpose just like you would do with an old laptop. If you still insist on replacing the part, you can always go to a "mobile hospital" (there are literally hundreds here in India, and I've seen one or two in pretty much every major US city) and get the screen, camera, or charger port replaced. Spares are available for everything. If you did the same replacement on a Fairphone, the most you would be saving on is the labor that the mobile repair guy would be charging. And even that savings would probably get lost because the Fairphone modules would be costly because of their low volumes, proprietary physical interface, and 'ethical manufacturing'. Sure, it's ethical for the big picture and Gaia would be so happy, but it's probably cruel to my communications budget which is already getting bled dry paying ridiculous prices for mobile data. $10 for 3 long youtube videos over your data limit? It's highway robbery out there.
As far as actual customization goes, the smartphone OS and hardware ecosystem pretty much has you locked in place. You can't really remove any features. Want a phone without a camera? you'll either have a half-functional phone (no QR codes, etc.), or you'll have a buggy piece of shit. Maybe the developer of your favorite app did the right thing and added a check for a camera, but most likely he just programmed it to go straight to the camera and getImage(). Want a phone without a GPS to go with your tinfoil hat? It's most likely coupled with some other useful part (like a modem) in a module, and even if you could take it out, the Google WiFi SSID-based location system's already got your exact location within 3 feet as soon as you turn on the WiFi. Want a phone with a bigger camera? Why not get the extra processing power that it would end up needing anyways and get the newest and biggest Samsung, HTC or Xiaomi. Wanna upgrade GPU, CPU, RAM or root storage? Fuggedaboutit - It's probably soldered to the base phone and/or inside an SoC chip. Want a new OS? Good luck finding anything that is worth switching to and will run reliably unless your phone sold at least a 100,000 copies. Want a bigger battery or a wireless charging system? Most good and recent-ish phones can be easily fitted with one.
You can't compare the IBM PC platform to these SoC phones, and here's why:
1. The choice of internal components is very limited, and a lot of stuff is combined into one chip. There are 3-4 major SoC vendors out there. Same goes for the Camera CCDs, sensors, and all the chips that go in there. Compare that to the ~10 major graphics card and motherboard manufacturers in the market. Or the ~40 different types of CPUs you can install on a given motherboard socket.
2. Unlike a desktop OS with replaceable drivers, an Android OS image has to already have all the drivers it needs installed in the image. With a desktop OS you get an installer that lets you install it on any compatible hardware. With a smartphone OS, the install takes a team of 40 engineers 6 months of work, day and night, after which they release the OS image (which still has fucking bugs in it, mind you, till they iron half of them out with every +.001 version). Or some smart kid makes the install using Cyanogen or something in one all nighter, but it ends up having bugs that never go away because the kid got bored of the project or got hired to that team of 40 engineers.
3. The choice of manufacturers and models at different price points is simply fucking crazy. There are easily hundreds of smartphone/tablet manufacturers out there if you know how to look beyond what Best Buy/Verizon will sell you. You can pretty much customize by simply choosing the right phone for what you need.
4. Unless your phone usage is very different from the average user, there's almost no difference in the experience of using any of these devices. No matter how good your phone is, it's gonna get slow as fuck anyways once you actually start using it because you'll keep installing shit until it slows down, and then try to optimize it. 5. When a good Samsung falls out of your pocket, you get a few scratches and your Gorilla glass screen survives the drop. When a modular phone does the same, the floor is scattered with modules and pieces of your dignity. Oh, and that 100 megapixel camera module that you got for $400? nowhere to be found - probably fell into the hole with the cockroaches in it.
Don't get me wrong - these problems could all be ironed out and we may indeed have a truly awesome modular phone in our hands someday. It's just going to a take an industry-wide paradigm shift (and a mature and widely deployed version of Project Ara) for it to be actually useful. Until then, the most a Fairphone will do for you is make you the star of one coffee break where people finally notice your Code Monkey ringtone. If you're really desperate and work at Facebook or some shit, maybe, just maybe it might be worth it for that.