golden goose 
noun
A continuing source of wealth or profit that may be exhausted if it is misused: they were killing the golden goose of tourism...See also kill the goose that lays the golden eggs at egg1.
- The Bells, regarding local services as their proprietary golden goose, successfully sued to gain control over such access and pricing.
- A domestic tourism market may not be the golden goose so keenly coveted by hoteliers, restaurateurs and others, but it is at least a solid silver or sturdy bronze one.
- Last week, as Amazon.com shares took a tumble, they must have been asking if their golden goose was about to be slain.