Wouldn’t it be nice to earn money while not working? That money is called residual, or recurring, income. It's what can happen after you put a lot of time, effort and sometimes money into a job to continue to get paid for the work months or years after it's done. (Salary jobs are part of linear income. This income is directly related to the number of hours you work. If you work 40 hours, you get paid for 40 hours of work.) Once you set up your business to earn residual income, you continue to make money while doing other things – maybe even starting a new business to generate more residual income!
However, residual income typically has an expiration date, especially if it is being earned through a business. Effort must be continuously put into the business in order for someone to continue to receive residual income. Businesses must continue to market themselves in order to remain relevant. The best way to look at residual income in this sense is that it is a part-time job that earns full-time income.
This is a fairly new concept to me still, but I now see that multiple streams of income are definitely far better than one high income stream as you point out. I now work part time as a hospitalist. That job provides our biggest income stream, but I also work part time for an opioid treatment center, which could easily turn into a full time position if I ever needed it (the owner has told me “you always have a job here”). Additionally, my wife works full time AND she runs an airbnb room out of our home which pulls in easily 1000/month. Those combined with our investments in stocks/bonds give us 5 sources of income. In the past we relied solely on my full time work and it stressed me out knowing that at any time something could happen that could take that away from us (as you listed above). Since I started doing these other things and we diversified our income, I feel much more at ease and relaxed.
The type of business you should start depends on your passion and existing skill set. If you love baking, for example, you could consider starting a home-based cake or brownie business. Love to sew? Spend your free time creating the perfect crafts, then turn around and sell them with your own Etsy store. Love dogs? Consider watching dogs out of your own home and marketing your services on a website like Rover.com.
Many people talk about passive income and create the impression that you never have to do anything to keep that income going. The truth is that you will normally have to keep your eye on things if you want it to run smoothly. For example Richard Branson doesn’t run any of the 400+ companies he started but he goes over the numbers each day to make sure they’re performing well and calls the CEO if there are any problems.