Why don't we say Yemach Shemo after people like Vespasian and other people who did bad things to the Jews many years ago? Why do we only say it after relatively recent Reshaim, e.g. Hitler.
Similarly, why don't we say Zichrono Livracha after Abaye and Rava, and just after more recent Tzaddikim? It is noteworthy that Chassidim ofter write a longer version of it after former Rebbes.