This doesn't make much sense because everyone transitions to new eras at different times, and wars can span entire eras, and transitions between eras.
There should be a more fine grained diplomatic response based on circumstances, did you declare, did you refuse peace, how much territory did you take, how many cities did you raze, etc. And even then there should be a decay on the diplomatic effect so in time even the most egregious warmongering in classical era should be a long forgotten memory 150 turns later.
Having it last one era is a huge step up from the entire game. You see the suggestion was not put there to fix the game, just to take it in the right direction. It boils down to the fact that just because your a warmonger at one point does not mean you should be penalized the whole damn game for making some mistakes. The hows and whys and specific rules that should be changed in order to fix this does not matter to me. The important thing to me is that it gets fixed. The whether too's and why for's are for those people, the developers to decide.