I think Lady Russell was more concerned that there wouldn't be a right time to marry. Remember that Anne and Frank were facing a long engagement of indeterminate length until Frank earned enough in prize money for them to get married.

And of course Frank could die in battle or of disease first. Or Anne could have met someone else.

And generally, once you get older and have seen how a few situations turned out in the past, you do start feeling urges to stop people making mistakes like you've seen in the past (thus encouraging them to make different mistakes, which at least allows for some variety.)