If a freelancer works just with 1 customer they should simply be hired.

A good strategy: 1 background job that is long running and has a reasonably low price tag because it can be pre-empted, one high priority, short bursts of work that are paid top rate and maintenance on a number of old clients that still bring in a part of the income.

That way everybody gets value for their money and the freelancer gets a good mixture of income. You're mistaken if you think that having a freelancer with only 1 or 2 customers is good for you, in the long term you'll need to rebuild your relationship with new people because your old ones will go out of business when you don't need them for a while.