We disagree in the first two sentences you wrote: "Hardware Wallets do not protect your Bitcoins. The hardware doesn't make your Bitcoins safer." Actually, a hardware wallet DOES make your Bitcoins safer than holding them on your everyday computer. Also, setting up and keeping your coins air gapped on a separate computer is a lot harder, more prone to errors and also more expensive than a hardware wallet. A Trezor goes by 70$. A separate laptop usually not under 200$. And how do you even keep the airgap if you want to send your coins somewhere? As soon as you try to transfer data between your bitcoin computer and your everyday computer (probably with a thumb drive), the airgap is broken. Dedicated malware can also be transferred via thumb drive.

You can definitely pass a PSBT file from any airgapped device but remember some of these devices aren't a panacea. Some people have had trouble sending large amount of inputs in one TX (140+) with some devices. The HWW couldn't absorb the PSBT generated to sign - legit failure. HWWs can also make your Bitcoins less safe by using their Random Number Generators & trusting that your device is legit & not tampered with. I do understand the use of HWWs for signing, but people store their seeds on these HWWs which are only protected with a pin code!! This is the point you've missed^