Also, I can imagine such schemes getting ditched for the same reasons of "now people can detect whether we already have a copy of the file, which is a privacy h

Also, I can imagine such schemes getting ditched for the same reasons of "now people can detect whether we already have a copy of the file, which is a privacy hole!" that killed cross-origin caching in browsers. Or if everything in the year 20XX gets pushed into using E2E encryption, since that's pretty much antithetical to deduplication.