I also doubt the percentage of startups that fail in a given year would compare well against the number of people that get laid off in a given year.
And having 100 customers doesn't help if losing 10 means the business is no longer profitable.
For most people, if something happens to their current employer, they can go get another one! In this economy especially, with a decent skillset you can probably go get another job in 2-4 weeks.
The real risk is much closer to _how many employers exist that might hire you_. If there are only a couple of businesses that need people with your skillset then that's a problem. If your skillset has a wide demand though then it's fine to only collect your income from one source at a time. If one employer dries up go take your skills elsewhere.