Modern and ancient word choice: 昔人 vs 古人 vs 前妻 Older Chinese folks (古人) love to use 昔人，but they never use 古人 in their poem or lyrics. (OR can someone give me an example where 古人 was actually using 古人 in their writing?) For instance, a favorite poem from the tang dynasty: 昔人已乘黄鹤去，此地空余黄鹤楼。黄鹤一去不复返，白云千载空悠悠。On the other hand, newer Chinese folks now rarely use the word 昔; instead, 古人 (昔人 old folks)，去年 (昔年 last year)，昨天 (昔日 yesterday)，前天 (昔日 the day after yesterday).
Why does the preference of certain word choices shift over time?
And since 昔 means 从前, technically, can we use 昔人 to replace 前妻/前女友？ OR does 昔人 have to be a 死人 (an already deceased person)？
never? you're kidding :) the term "古人" is also used in poem.

just a casual search: 

in 通典·卷三十二 (end of p73, and start of 74):


  賦詩曰
  
  古人云此水﹒一飲重千金
  
  若使夷齊飲﹒終當不易心


next, 杜工部﹒ 吾宗


  吾宗老孫子﹒質朴古人風﹒耕鑿安時論﹒衣冠與世同
  
  在家常早起﹒憂國願年豐﹒語及君臣際﹒經書滿腹中


later, 孟宗獻 wrote


  古人借宅亦種竹﹒大是饕竒心未足﹒髙齋聞有萬琅玕﹒坐對懐山飲秋緑﹒
  
  官閒勝日無一事﹒尊酒不空仍有肉﹒他時剥啄叩君門﹒髙枕矮床容我宿


imo, characters are like fashion, "trendy" today, would out-of-date".

have fun :)