] When first Mr. Bennet had married, economy was held to be perfectly useless; for, of course, they were to have a son. This son was to join in cutting off the entail, as soon as he should be of age , and the widow and younger children would by that means be provided for. [emphasis mine]

The point is that only the heir (ie the hypothetical son) could do this, since he would be the person who was giving up some rights. Even if this son did not agree, then, unless he was singularly undutiful, he would provide for his sisters and mother.