Let's hope you won't find out why that's a very bad idea.
> Clearly having multiple customers isn't a reasonable requirement for a small company.
On the contrary, it's a must.
> Software engineering isn't like private residential plumbing - "sink drain unclogged, next please!" :)
That's a strawman.
Sorry, but if you have just one customer and you're developing software other than taxes and your rights you are less than an employee. Don't kid yourself, that tends to lead to rude awakenings. If at the end of the year you've only sent invoices to a single customer then you are simply at risk. You need multiple customers to be stable and secure. Two is better than one and three is really the minimum.
Loss of income insurance is to take care of mishaps, not to insure against market downturns or other normal risks that a business is exposed to.