I agree with the ideia of the hardware wallet is just a gadget with a tag of "there is bitcoins", but talking about the security of the hardware i think the worst problem are not the external attacks of hackers breaking the firmware and stoling the bitcoin, I think the problem is with the trust of the companies which sell these wallets, most of the avaliable wallets in the market are not open-hardware and doesnt have the firmware open for the public, there is only apps which the public can help to develop.
Technically, the same thing could happen to your trusted software such as Armory, Electrum or Bitcoin Core. Not so long time ago, there was a critical vulnerability in Electrum which allowed to steal your wallet using JavaScript. It shouldn't be a problem on air-gapped laptop but still there are probably more bugs that we do not know about.
Furthermore, hardware wallets are not just generic storage devices, so a computer infected with malware can't read or write arbitrary data to a hardware wallet as it would with any normal storage device.
I'm quite interested how TREZOR will handle it. TREZOR T has a micro SD card slot which could be used for something malicious. Right now, it is only used for updating the bootloader at startup. Encrypted storage was also mentioned but I guess it will be separated from the rest of software and hardware.