Wait. Isn't it painfully obvious when you've been simjacked? If your phone suddenly loses signal and refuses to register with the network, you know something is up. You may think it was a malfunction of your phone or your network, but it's pretty much a definition of a modern-day "drop everything you're doing and deal with it" emergency. You can't not be aware of it, or be unsure if it happened to you.
You very much can not be aware of it. Consider what happens when you're simswapped at 2 am. Are you going to notice that? Probably not. And maybe not after you get up and check your phone. Because your phone may be connected to the internet via your home wifi and you don't even notice your phone has no bars and no service because you're still able to browse the web and check email.
But if the attacker already has your info, then couldn't they just add another line to your mobile plan, so your handset continues working, just with a new, unbeknownst to you phone number? That way it wouldn't be noticable on the handset.
The real question is how long do you think it would take you to break into your own Gmail account after the passwords been changed and the attached phone numbers also been changed?
Probably longer than it would take an attacker to drain bank accounts, I figure.
By the time you notice and can react it's too late. There have also been many prominent examples of people who got their cryptocurrency exchange accounts broken into with SIM hijacking which was conducted while the victim was asleep.
