The issue is not removing old pipes, but even getting access to install new pipes. This typically means opening up walls and floors, and patching the resulting holes after it's done.
Yep, this was the longest part of the process. Plumbers were in and out in 3 days but left a huge mess to deal with, which we were aware of.
Yes, a lot of the old pipes were left in place. That seems to be fairly common as they are difficult to remove given they are made of copper