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)？