Replace SubDevice by Device references / support bottomless device hierarchies?

A question that is asked from time to time is why SDT is limited to one level of SubDevices. We had this discussion in the HGI at that time and found no real good requirement to have a non-restricted device hierarchy. Yes, in theory it is nice to not be limited to a two-level device hierarchy, but in practice I have yet to find this a real limitation.

The question is whether we should take the opportunity to change this or whether the current solution is still good enough.

One possible change could be to make sub-devices first-class citizens / devices, and then reference those devices from another device.

What are your opinions about this?