While this is true in general, I am amused that this is not true for citing wikipedia. Wikipedia can be trusted to remain online for many more years to come. And it has a built-in wayback machine in the form of Revision History.
The page can be completely correct and accurate, but if you cannot trace the references then it cannot be verified and you cannot make the claims in a new work as a result. The whole point of references is to make it so that the claims can be independently verified. Even when there isn't a link rot problem you will often find junk references that cannot be verified.
Wikipedia isn't a bad starting point and sometimes you can find good references. But it is not anywhere close to reliable: just trace the references in the next 20 Wiki articles you read and your faith will be shaken.
Good quality information on wikipedia often refers back to published sources, and at the very least an author should check that source and refer to it, rather than wikipedia itself.