Remember that Captain Wentworth and Anne were already looking at a long engagement until he'd earned enough money for them to marry on (or one of them had conveniently inherited enough money to marry on) before Lady Russell interferred. They would not have married and risked having children to bring up in poverty.

Lady Russell thought the long engagement itself was a bad thing. Anne would be exposed to all the stresses of waiting for him for an unknown period of time, and unable to meet suitable men who might have been able to marry and make her happy on the spot. Of course she could have broken the engagement, but breaking an engagement after three years on the basis that "I found someone else" or "I'm bored of waiting" sounds rather dishonourable even nowadays.

Perhaps it would have been more honourable, given the risks of long engagements, for Captain Wentworth to have refrained from proposing at all until he had sufficient fortune. But I think a man madly in love can be forgiven for seeking some proof that his affections are returned, and the pleasures of being able to write and receive letters during all the times that they must be separated.