Maybe that is the case but they only have security updates for few years at most, I think apple has 5 years, samsung flagship devices has 4 years, cheap phones 1 or 2 years, and than they become unsecure.
Since Airgap wallet is based on older device, that means that most of those devices are generally unsecure.
Yes, old models are less secure than new models, but nothing stops the user from using a new model. We started with the idea "based on older model" but some of the libraries we use now require users to use more recent OS, which in turn requires them to use new models.
So is there a way to revert changes to default state with Reset phone function, and remove AigGap later if we changed out minds?
Yes, there is a factory reset button. After factory resetting, your devices comes back to normal.