Hm, I think in this regard ::std/::crate proposal is isomorphic to variant 1? Like, in V1 we deprecate use std::foo and require to write use extern::std::foo. In ::std/::crate we as well deprecate use std::foo and require to write use ::std::foo. The :: plays exactly the same role as extern::, but is strictly shorter 
Am I correct that we don't want to lean on epochs to make a breaking change here, because "to upgrade to a newer epoch, you must change every use statement" is unacceptable?