I wonder why the iMac still has that chin design and small panel. They’ve been producing displays for long enough that the iMac and Studio Display should have the same form factor.
The chin is where Apple puts the logic board and speakers for the iMac. Yes, they could move that behind the display, but I'm guessing that'd make the display thicker (which they wouldn't want) and make the thermals harder. It's certainly something they could engineer, after all the iPad has its logic board behind the display. But it's not like the chin serves no purpose and it probably helps with the thermals and sustained performance and probably makes it easier to engineer everything for what is probably one of their lower volume sellers (compared to iPhones, iPads, MacBooks).
Makes the angle very easy to adjust.
I used to put post-its on mine too.
There may be some technical reasons for it, but I'm not convinced. The chin is a core part of the imac design language. Without the chin it would just be a display and not recognizably different from any other display. The chin is part of the design that that makes it recognizably an imac.