Postponing an engagement means withdrawing from it with the chance to renew it at a later, more propitious time. It is not entering a long or uncertain engagement, which was Anne was originally in for. Getting to know Capt. Wentworth better is still an option, but Lady Russell will not.

I agree that an engagement can be broken, and that sometimes it would be best so. However, my point was that Anne has made a decision, and Lady Russell is riding roughshod over it.

She convinces Anne that the engagement was not deserving [of success]! Parse that sentiment: if there had been success, if Capt. Wentworth had gone on to make a fortune, and Anne had married him, the marriage would be undeserving? Their happiness would still be unworthy of existing?

Lady Russell never wanted the past undone, even when she sees Anne early lose her bloom over it, nor when she is despairing of Anne finding a home of her own. No matter what, Anne's happiest episode is still a time of wrongness to her best friend, and Anne's current state is preferable to Wentworth's proposal having been accepted in the first place, regardless of the outcome. Even had it been as successful as his rapid rise of fortunes would have made it.