I just explained why it doesn’t matter that “most” people can manage their OS, that’s because Google has thousands of Linux users. If 1% of Google developers make a critical mistake that could be a hundred workstations or more.
On top of that, I still disagree. Many developers I’ve met are not that good at using the OS.
Remember that writing applications is a completely separate skill. You can learn to code completely separate from learning anything else about the computer. I had a college professor that taught C on a chalkboard, in this millennium.
Google’s own developer interviews are very CS and algorithm heavy and to my knowledge never test your abilities with OS management.
When you say “really just needed for mobile devices including laptops” that’s basically all devices isn’t it? You’d have to be incredibly specialized to need or prefer a desktop workstation.
I haven’t even brought up compliance controls and certifications yet, either. If you’re SOC 2 or HIPAA compliant you have to manage workstations to disable functionality and restrict system preferences. For example, there are typically restrictions on USB port data transmission and idle screen lock delay time.
For your probabilities, you describe many issues that aren't big issues at Google. Phishing credentials is mitigated by mandatory FIDO tokens, binary whitelisting heavily reduces installs of malware, security policy isn't needed on these desktops (really just needed for mobile devices incl laptops).