I think they took a giant **** on HK,T3,Meetra and Revan too but the only difference is Revan is the only one that can be saved because He didn't FOR SURE die on the foundry he just got DEFEATED so his character can be saved if they did something epic like my idea. They cant save the other 3 because they KILLED them off, there's nothing that can be done. I understand what your saying about leaving characters in the past but Bioware DIDNT leave them in the past they brought them back.
Droids are easy to save. T3 copied his memories into a T7 model as backup. HK would be more likely to backup than T3
As for Jedi/Sith they never die, They come back as force spirits and screw with the universe for years