I think the modern idea of a "cloud" is the Plan 9 philosophy coming full circle. Many breakthroughs in Plan 9 (distributed architecture, everything as a file, unified user experience) are being recreated on the web (cloud storage/photos/music/email, open APIs so all your webapps can intercommunicate--when I squint really hard while looking at screenshots of Plan 9, it almost reminds me of google's suite).

The OS will become extinct once everything happens in the web browser, and the OS becomes not much more than a web browser itself. And then they'll have pretty much reimplemented Plan 9.