In Latin America people kidnap you and take you around to ATMs at gunpoint until you've reached the withdrawal limit. This seems so much more dangerous with bitcoin wallets.
There are wallets that will show a fake lower balance in distress mode.
Further, why walk around with all of your BTC available to you? If you really have enough to worry about gangs stealing it, put it in cold storage or on an exchange, but don't carry access to it around in your pocket the same way you wouldn't carry cash around in your pocket.
A fake UI screen? Wow, that's useful. Good thing there isn't an easily verifiable public ledger with the true value of the account that the criminals can check while holding you at gunpoint.
Meanwhile, you expect people to have cold storage where? IIRC, they cannot just memorize a private key, but they need an actual file on a device.
