As a consultant these past 12.5 years, making a high income, I completely agree with almost everything you said. My experience however is that a single client, if it's the right client, can be enough.

Having said that, it can be a high risk high reward scenario. If that one client goes away, you have to start over almost from scratch.

To make this survivable, you have to invest a large portion of your income, keep a few small side efforts going (so not purely single client, but 90% counts I'm my book as single client), and make yourself indispensable.