Back in 2011 when 2011 iMac was fresh? You can't. Now? It's possible.
But again, I wasn't debating about iMac spec-wise. We're talking about form factor here. iMac spec is a good deal for the money. But AIO brings a notebook limitation to desktop machine.
Then I ask you again the same question. IF Apple sells a quad MacPro + 27" Cinema Display for $2000, would you still pick the iMac over it?
Case, CPU, GPU, HDD, Ram, Ethernet adapter, I/O interfaces, and etc for only a $1000?
Still think that would be tough to pull off now. Base 27" iMac has quad core i5 CPU, no? What would that cost?
The only mobile limitation seems to be the GPU so unless you are a gamer looking to play the latest games at max settings this isn't a huge limitation.
Seems to me the only users that require shorter term ability to upgrade are gamers.
Hardcore gamers don't really benefit by buying a Mac anyway.
Btw, I have a Mac Mini connected to a TV and a MacBook that is used with a Cinema Display.
Not really an iMac fan but understand why some users prefer the design.