How can you talk about security if there is spyware on every Chromebook? I mean Google Chrome. Everything you do is recorded on Google servers, not only that but Chrome doesn't integrate any anti tracking protections like other browsers do (even Chromium based ones like Microsoft Edge), because it's the interest of Google to track the users. Not only that, but Google Chrome has a lot more security vulnerabilities than the competitors. I wouldn't say it's secure, and I would never do something on a Chromebook or in general in a Chrome browser, since it's like giving the access to Google itself.
Regarding the operating system, how can you trust more a system from Google than a system you build yourself? If you are paranoid you can buy a machine that supports Coreboot and even run your own trusted BIOS. If you don't want to take this seriously, you can setup your secure boot by creating your own keys, installing them in the firmware, and signing the kernel that you compile yourself. I don't know why you would have to be that paranoid but whatever, to me when I have an encrypted hard drive and my computer stays in my bag is more than enough.
> Almost everything can be done on the web today
You can't do programming in the browser, you can't do graphics, video editing, 3d modelling, you can't connect to a remote computer with RDP from a browser, you can't even have a full office suite in the browser (Office 365 in the browser, Google Docs, etc are limited), you can't play big games in the browser, you can't connect to a VPN from a browser, a webmail it's worse than using a desktop email client, you cannot have an advanced video/audio player in the browser (like VLC), etc.
Also, if internet doesn't work you are basically screwed, what do you do? Stare at the screen and play with the dinosaur game?