The engagement, already entered into, could not be "postponed". Only the wedding could be. The engagement could be broken and re-entered into later, and CW was free to propose again when he had gained independence in a couple of years- but he did not choose to do so.

Anne could not tell FW that she was breaking off the engagement but would accept him at a future date if he made a fortune and proved his claims- that would be indelicate, mercenary and selfish. It would have been best if FW had put off proposing to Anne until he had gained his fortune, leaving Anne free to become engaged to another man if she wished to do so (Would Anne have accepted Charles Musgrove had CW not proposed to her?).

In stating that She was persuaded to believe the engagement a wrong thing -- indiscreet, improper, hardly capable of success, and not deserving it. it means the engagement was hardly capably of successfully culminating in a marriage (it would be broken off eventually), and that even if it did succesfully culminate in marriage, it would not be a good thing. I don't see it as meaning the success of the marriage itself. At least, that is my take on it. :-p

If CW had not gained a ship and fortune as rapidly as he did, how long would Anne have had to wait? Two years? Three? Five? At the end of it, FW or Anne having grown weary of it, the engagement may have been broken off. Breaking an engagement after such a long time would taint Anne's reputation.

If Anne and CW had married immediately, they would have no financial support either for themselves or for the children that would follow. I can imagine Anne succumbing to a fate similar to Mrs. Price in MP- a youth-killing dependence.