I am far too lazy to tag every file and I would despise such a system if forced into it. Some files I want to keep realizing I may never need them again. Tagging is a time waste. It's also difficult to anticipate future use and what tags are helpful.
Now a tagging system that could be built over time from search results could be very useful. Apply tags as you go in batches in other words to aid future searches.
If I were designing a new OS, I'd force each piece of software to auto-tag and index the file contents in meta tags and feed it to a global OS search function.
You assume that you need to tag manually. You assume that you are forced to do so. Currently, it's perfectly possible to put all files in a single folder. Your OS will be happy about that (except maybe some technical limitation of max filecount of a folder).
Now a tagging system that could be built over time from search results could be very useful. Apply tags as you go in batches in other words to aid future searches.
If I were designing a new OS, I'd force each piece of software to auto-tag and index the file contents in meta tags and feed it to a global OS search function.