It would be impossible to do. Even back in 1991 when unicode was conceived almost all the encodings in use were ASCII-compatible.
For languages that would be affected by a greco-unification that meant the encodings that were in use before unicode had both the latin script and their "national" script.
Implementing greco-unification in unicode would mean that round-trip lossless conversion (from origin encoding to unicode back into origin encoding) would be impossible, greatly limiting unicode's adoption.
No such problem existed with han characters, in fact JIS X 0208 (the character set used for Shift-JIS) did a very similar thing to unicode's han unification.
In absence of backwards compatibility problems I would be in favor of greco-unification too.
For languages that would be affected by a greco-unification that meant the encodings that were in use before unicode had both the latin script and their "national" script.
Implementing greco-unification in unicode would mean that round-trip lossless conversion (from origin encoding to unicode back into origin encoding) would be impossible, greatly limiting unicode's adoption.
No such problem existed with han characters, in fact JIS X 0208 (the character set used for Shift-JIS) did a very similar thing to unicode's han unification.
In absence of backwards compatibility problems I would be in favor of greco-unification too.