ALL YOUR EGGS IN ONE BASKET
As a mom, I was picky about who babysat my kids. There was only one girl I really trusted with the kids, and if she was unavailable, I was kinda outta luck. It's not that there weren't more babysitters that could do the job. It's just that I was probably kind of comfortable and didn't go beyond my little neighborhood to search for other reliable babysitters that could successfully do the job in case of an emergency. Until that one day, when, (you know how the story goes) when my babysitter was unavailable, yet this was the day I really, really needed one. That was the day I knew what the saying, Don't put your eggs all in one basket" really meant.
We've probably all been in the position of counting on one person or thing to do something for us, riding in a carpool, going on a vacation, flying a certain airline, relying on only one vendor. But the problem with that, is when you get comfortable and think everything is going along smoothly, when that other person or thing shuts down, quits, changes their algorithm, ups their prices, changes their mind, goes on strike, drops in the market, we are the ones left holding the empty basket.
The business lesson is this. Don't rely so heavily on one person, thing, company, service to make your business go. (or your life either for that matter) You've got to have your hand in several pots and constantly evaluate - and maybe even have a plan B. Planning and knowing what you will do if something happens to that person, thing, company and they are no longer available, will help your life be less stressful.