While we tend to add less new Obj C, we don't go out of our way to replace old Obj C with new Swift if the old code is working. If it makes sense to replace some of the Obj C, we do, and this happens on a case by case basis.
At the end of the day code is just a tool, and if it's doing its job it's worth a sober look at whether the opportunity cost to replace it is really worth it beyond some perceived notion of newer == better.
Some things that I really like about Obj C more than Swift:
easier to explicitly manage memory
clear separation of declaration and implementation files
faster to compile
easier to interop with C
Resources: Obj C is old, and there's a ton of info out there on how to solve your specific problem