The problem with existing OSs is that if you don't make it the default and force everybody to adapt you end up with a half-assed solution with bad support that

The problem with existing OSs is that if you don't make it the default and force everybody to adapt you end up with a half-assed solution with bad support that just gets in the way. I tried to use firejail to sandbox critical apps (mainly my browser, but also the closed source Spotify client for instance) but eventually gave up because it would just break too often. On top of that the one app I'd like to sandbox more is obviously the web browser, but since nowadays web browsers are like a full OS on top of another it's really hard to finely grain permissions unless you want one profile per website. The browser needs access to the filesystem if you want to upload and download files, the graphics card if you want good performance and webgl, the sound API if you want audio, the ability to fullscreen if you want to be able to play games and videos that way, the printer if you want to print stuff, the USB stack if you want to use security tokens etc...