Whoops, my comment isn't very clear, sorry. I meant: "but their account would still be active and in the hands of the thief, if there is no way to quickly deactivate it, e.g. before receiving a new SIM card from their operator that would enable you to prove your identity to WhatsApp."
Do you mean how long is account recovery by the SIM/number owner possible, or how long can the phone thief continue using the WhatsApp account if the owner doesn't recover?
Maybe I misunderstood the comment you and parent comment were making. I interpreted it as "they can recover it via SIM, so the lockout method isn't needed".
My point to that is that it is true, but the lockout would prevent a thief from using it until the new SIM is received. Versus a thief having access until the new SIM is received.
I use telegram instead of Whatsapp, but I would hate for anyone to have any time at all on my account. I'd prefer to immediately lock the whole thing down and figure it out once I have everything sorted.
Can't the legitimate owner recover the account once they get a replacement SIM?