While I do understand the benefit of this it really is just taking a console and turning it into a PC. Which is fine as long as every upgradable part is compatible every other upgradeable part as well as the OS. But really that's just PC gaming presented by Sony, it isn't a console anymore.
There are two roads you can go with something like that though (PC gaming is one), the other is upgradable bits made by the console manufacturer. A la Sega 32x, Sega CD, among others. These things tend to not be cheap and based on that few people buy them. Because few people by them, few game devs support them.
So yeah, while it would be cool to be able to pop more ram into the ps3 and a new graphics card and things like that, at some point pretty quickly it stops being a console. The other problem with this is the warranty, I know Sony is ok with throwing a new harddrive in, but I'm not so sure they would want everyone to be able to open it up and replace the processor or things like that.(Not to say I'd really mind if tehy did this since if they did a 10 year life cycle would be fine since essentially the only things obsolete by the end of the life cycle would be the board and chipset, maybe the processor due to the above warranty issue.)