Actually, I believe the safest OS 'for the masses' is not even a Desktop OS at all. Most people will be most secure using a tablet / iPad with a keyboard, probably. All software will need to come from the official 'AppStore', everything's sandboxed, there will be no drivers, no privileges, no .exe's from the web. These days, you can easily work on Microsoft Office documents and manage your photo / video library on a large iPad.
I think the problem with this approach is that it almost encourages users not to pay attention to their security and not to do any due diligence. We see countless fake wallet apps popping up on various app stores, and users downloading them and using them without a second though because they wrongly assume Apple or Google have done their due diligence for them.
How's that? How can you infect my machine by sending me a .txt file?
If it is definitely just a .txt file and you only open it with notepad or similar, then you will almost certainly certainly be safe. However, I might send you a file called pgpkey.txt.exe or pgpkey.txt.bat, and since Windows hides file extensions by default, then it would show up as pgpkey.txt. You double click it and the virus runs. Or maybe I send it you as a .zip or a .rar which you think only contains a .txt file, but has other hidden files in there too. On Windows, I could create a shortcut to some malicious web address and rename it to pgpkey.txt and send it to you. You double click on it, and your browser opens the malicious link. I am by no means an expert, but I'm sure there are countless other ways to disguise a virus as a .txt file.