2TB it's enough. then buy external drives
I get your point about there being other ways to expand storage capacity, but there are trade-offs to that.
Probably the most incongruous is that the iMac design is all about a clean, lean, uncluttered look. You have the screen, a single power cable, a wireless keyboard and mouse. That's it. No other wires or even buttons visible. If you start adding external drives, now you have clutter that destroys the look, takes up space, needs to be plugged-in somewhere and isn't as fast as on-board storage.
An option is to use cloud storage, of course, but now you're having to add a NAS or, in my case, a second NAS (because you want your files backed up on a different set of disks). It might just make sense financially given the cost of boosting the SSDs in the new iMacs, but then you have the increased traffic on your network which might not bother everyone but it's an issue for me; I store TBs of media on my iMac's HDD to watch on my Apple TVs, and pushing a 20GB movie through the WiFi from my Mac uses a chunk of bandwidth. If iTunes had to call it from a NAS first, then you're doubling up traffic.
There are workarounds to all of this, I know. But they cost money, aren't elegant or simple, may not be practical depending on your set-up, and go against the entire "Just Works" mantra of Apple. That last one being my most significant frustration.