@wereinshades - I just happened to run in to one of their recently retired bosses and asked him about it. That was what he told me - it sounded to me like they had coverage issues with their system or upgraded system and my impression was that it was starting to look like a money pit. It was a brief informal conversation, so I can't provide you specific costs or details as to what drove them to make the decision.
I did do some digging and found they bought 100 APX Next radios through the Cook County ETSB specifically for the purpose of going to the Cook County Radio Network in June 2023 (begins on page 41):
https://cms6.revize.com/revize/tinleypark/document_center/Agendas and Minutes/Committee on whole/Agenda/2023/062023 COW.pdf
As far as cost savings, that seems like a lot of money up front for radios, but as far as I am aware if Cook County doesn't charge an agency if they have an agreement to use a talkgroup on their system as long as they dispatch for themselves since their radio system is funded by UASI grant money. The threat always being that if the grant money ever dries up they will have to start charging subscriber fees, which as far as I am aware, has not happened yet. If I was looking to escape the long term costs and headaches of maintaining a radio network, especially if you struggle with coverage issues, it would be a viable option to me to make a change like that. But again, I can't speak to the specific details or costs that drove the decision.