How do people get around Paris when transit employees don't feel like working that day?

They don't, because that would be the whole point of a total shutdown in a coordinated, all-modes transit employees strike. Ask people in London, they have that on a semi-regular basis.

Otherwise, there is no such thing as "transit employees not feeling like working" - thanks to the magic of economy holding a metaphorical gun to the heads of most people. You work whether you feel like it or not.