I think the point is that as a user you expect revocation of trust to protect you going forward, yet it doesn’t (e.g. the server shouldn’t be forwarding new messages to). That’s a design decision Signal made but clearly it’s one that leaves you open to harm. Moreover, it’s a dangerous decision because after obtaining the IK in some way (e.g. stolen device) you’re able to then essentially surreptitiously take over the account without the user ever knowing (i.e. no phishing needed). As an end user these are surprising design choices and that Signal discounted this as not part of their threat model to me suggest their threat model has an intentional or unintentional hole; second-hand devices that aren’t wiped are common & jail breaks exist.
This isn’t intractable either. You could imagine various protocols where having the IK is insufficient for receiving new messages going forward or impersonating sending messages. A simple one would be that each new device establishes a new key that the server recognizes as pertaining to that device and notifications are encrypted with a per-device key when sending to a device and require outbound messages to be similarly encrypted. There’s probably better schemes than this naive approach.
This isn’t intractable either. You could imagine various protocols where having the IK is insufficient for receiving new messages going forward or impersonating sending messages. A simple one would be that each new device establishes a new key that the server recognizes as pertaining to that device and notifications are encrypted with a per-device key when sending to a device and require outbound messages to be similarly encrypted. There’s probably better schemes than this naive approach.