In what world are these apps non-invasive? The kind of things they're intended to do are inherently invasive.
Also, I really don't like this scenario:
> Paul needed to turn off Gatekeeper to install an app he found on the Internet, then failed to turn it back on again, and his Mac got infected with malware.
Just turning off Gatekeeper can't result in your Mac getting infected. You'd also need to choose to run a malicious program. Think of it this way: Linux doesn't have any equivalent to Gatekeeper, but it's not less secure than macOS is.
In what world are these apps non-invasive? The kind of things they're intended to do are inherently invasive.
Also, I really don't like this scenario:
> Paul needed to turn off Gatekeeper to install an app he found on the Internet, then failed to turn it back on again, and his Mac got infected with malware.
Just turning off Gatekeeper can't result in your Mac getting infected. You'd also need to choose to run a malicious program. Think of it this way: Linux doesn't have any equivalent to Gatekeeper, but it's not less secure than macOS is.