Well, if I assume that I just got temporary access to someone’s unlocked device, then it would probably be a lot more convenient for me to quickly transfer the account to one of my own devices and then access it from there instead of accessing it from my targets device which I might lose access to any moment.

So from that point of view it would be legitimate to argue that I might want to get notified if one of my contacts transfers his account. I can then double check : “Did you just transfer your signal account to a new device or was that an attacker?”

That might only be interesting for high-risk users though and could impair the UX. Why not make it optional?