There are a certain number of things you just can't have a backup for - I've had a frame break while riding it (bottom bracket rusted out on an older steel frame). If you bend your front wheel in a minor crash you can't ride it. Etc.
Uber/Lyft is my backup plan for these kind of emergencies. It's not feasible to carry a spare wheel, a spare frame, or a welding torch with me, lol. I carry equipment to fix a flat as it's often more convenient to just do it myself on the trail.
My dad has an extra bike so if he wants to go riding on one bike is having issues, that's what I would do if I was wholey reliant on a bike for transportation. I already have a bit of space issues with an apartment and different kinds of bikes though (road, mountain, winter) so keeping a backup bike isn't worth it for me.