None of this is wrong, but there are a couple of nuances to add:
1) In looking at their respective financial positions, this analysis leaves out the support that they can expect from Mr. Gardiner and Mr. Philips (Mrs Bennet’s brothers.) If Mr. Bennet dies and his heir throws them out, they have relatives who can and almost certainly will take them in. This is something the Dashwood ladies do not have, by the way, or at least not to the same degree; Sir John Middleton may have let them take Barton Cottage on very easy terms, but they are still renting it from them, and they do still have all the rest of the household expenses to bear.
Mr. and Mrs. Philips don’t seem to have any kids (no cousins are mentioned) and we know Mrs. Philips loves Mrs. Bennet and the Miss Bennets. Mr. and Mrs. Gardiner do have kids, but also, Mr. Gardiner’s business is apparently quite successful: he has the time and money to take long vacations, and he and his wife dress in such a style that people who don’t know them (i.e. Mr. Darcy when they meet at Netherfield) take them for “people of fashion” i.e. members of the gentry class who have wealth but not land and hang around going to lots of parties. Between the Gardiners and the Philipses, they can afford to support the Bennet women. Most likely, Mrs. Bennet and Lydia (and maybe Kitty) would stay in Meryton with the Philipses, and the Gardiners would take the rest of the girls. (Mrs. Bennet and Mrs. Philips are close, and Lydia is a favorite of both Mrs. Bennet and Mrs. Philips.) In London, the older girls would help out around the Gardiner house and attend social events with them; they would be exposed to a lot of men ‘in trade’ and would have a better chance of marrying than in Meryton (where there are only four-and-twenty families they have social contact with, and if there were any young men in that group they were willing to marry who were willing to marry them, we would have met them in the book).
The Miss Bennets would probably marry either professional men or men ‘in trade’ and sink back into the middle classes their mother came from; they would not be poor. They would be dependent relatives until that point, but they have good family who love them who will allow them to be dependent and not treat them badly for it. It’s not ideal, but it’s pretty far from the “starving in the hedgerows” that Mrs. Bennet wails about. So when evaluating “is Mrs. Bennet right or not” that is something we need to keep in mind.
2) One of Austen’s main concerns is not just for her heroines to marry but to marry men who will treat them well. There’s always a jerk/rake who looks awesome on the surface but is actually awful. In P&P it’s Wickham; in S&S it’s Willoughby. Austen didn’t think love necessary for a good marriage (consider Charlotte and Mr. Collins, and Charlotte being quite content). But she did think that “knowing his character and knowing he’s a decent human being” was critical. That’s why the ‘boring/stodgy’ guy you’ve known all your life is a better catch than the handsome charmer you’ve only just met. That’s why Elizabeth begins to fall in love with Darcy when she sees Pemberly–not how wealthy he is, but how well he treats the people in his power, his servants and his sister. Austen never dwells on this, which is why so many people miss it, but it’s a constant undercurrent. Then, even more than now, there were a lot of women who were deeply unhappy–and often suffering–because their husbands treated them badly and they have no escape. Austen expects her readers to know this, when they’re evaluating the potential love interests in her books.
Mrs. Bennet, crucially, does not know this. Mrs. Bennet wants her daughters married, and she doesn’t care to whom. She does not care about the character of any prospective son-in-law. She does not care about whether it is possible for her daughter to live with him without hating him. She doesn’t even really care about whether or not he can afford to support a wife, or she wouldn’t be so thrilled with Wickham (who has no money and no prospect of getting any–army officers, whether in the militia or in the regulars, were deliberately not paid enough to support the lifestyle of the gentlemen they were supposed to be, because it would keep people who weren’t independently wealthy out of the officer corps).
Marriage to a rich man would save the family financially, but it’s not worth it if the woman so married is miserable for the rest of her life. And, given the family support that they can expect, this is extremely short-sighted of Mrs. Bennet.