That risk is not just theoretical. I made a test account (on another service; not Signal) using a free anonymous SMS number. A few months later, the account had been hijacked.
Of course, if it’s a disposable account, then having it hijacked after you’re done with it might be a good thing.
Register a new account over that phone number.
They can’t get into any previous accounts register with that phone number.
They could potentially manage to find the pin if the previous user really used a guessable one, but then again, they won’t be able to check the previous messages and the linked owner of that account will be warned of that new connection.
That risk is not just theoretical. I made a test account (on another service; not Signal) using a free anonymous SMS number. A few months later, the account had been hijacked.
Of course, if it’s a disposable account, then having it hijacked after you’re done with it might be a good thing.
Signal has account pins now so I don’t think the attack vector is as large as it used to be
They can’t “take over” your account, but they can “override” it and delete yours.
How can they override it?
Register a new account over that phone number. They can’t get into any previous accounts register with that phone number. They could potentially manage to find the pin if the previous user really used a guessable one, but then again, they won’t be able to check the previous messages and the linked owner of that account will be warned of that new connection.
I don’t think that’s possible with a registration lock unless you are inactive for longer than 7 days.