It would be great if we were able to customize cameras just like we customize PCs, but it's not happening yet.
You mean it's not happening anymore
(or less than before).
Medium Format have had interchangeable lenses, focussing screens, viewfinders, film backs, digital backs, full meters, spot meters, you name it, pretty much since they started.
One i know of is Hasselblad ditched the ability to swap film/digital backs when they introduced the H3D (although apparently you can still use the waist-level finder).
Other MF companies still produce digital backs and bodies seperately, but the trend is still more towards integration.
afaik, 35mm has never had the same level of modularity.
ok, so angle finders are available, but they clip onto the back of your viewfinder, they don't replace the entire prism.
has any 35mm ever had a replaceable back? (granted, early DSLR digital backs tacked onto film cameras, and they're not made anymore either).
and replacing your ttl full metering prism with your spot metering prism disappeared with the berlin wall (now i just press a button and scroll a wheel to change, i can tell you what i prefer too)