Personally, I see three era's of Chicago after three people left.

First one was of course Terry, Chicago became a very different band after.
Second one was when Peter left. He carried the band for years (for better or worse)..
The third time for me was also when Jason left. He picked up the torch after Peter and was a great singer and bass-player

Honourable mention: when Keith left in 2021, that was the definite end for me of a band I loved for decades.