Everything in my home can still be switch operated. Only the "convivence" items such as the evening light modes along with the house is dark light modes wouldn't work. If the basement lights don't come on when the stairwell switch turns on they can still use the pull chains to turn individuals lights on and off.
My backup is my son. He has been working with me in the tail end of this journey. Even though I don't make any more daily updates I still like to keep on top of Hubitat updates.
The bigger questions is the folks that I support at other locations. These are mostly hands off and very few that even know how the system works. I work with some "partners" that have done electrical or networking that can handle the simple task to tell them to reboot the hub but if say a Z-Wave device fails and needs replacement they wouldn't know what to do. But again even at those locations everything can still be operated manually.
If someone wants to take over they can look at the development notebook I made that outlines the automations and some of the configuration. It's all stored in a excel workbook and easy to understand the logic. Where they would have issues is looking at the Hubitat documentation and determining how Rule Machine works and some of the apps. I provide this notebook to all my clients.