Well, cash was hardly the only way they had to provide for younger sons. For all we know Darcy may have owned (or could acquire) other properties besides Pemberley, and at the very least we know he's got a number of valuable livings at his disposal which could be given to a son. Then he could assist other sons in other careers, such as politics or the law. John Knightley is a lawyer, and obviously does very well for himself. Some barristers made as much as twelve thousand pounds a year in income. So, really, if they just set aside a small fortune for younger sons, enough to generate a few hundred pounds in income, that would be gravy, giving them padding to make sure they're comfortable while establishing themselves in a career.
As for dowries for daughters, those could be promised in future as well as present--this much money now, and additional money at regular intervals for a specified period of time, that sort of thing. I agree it's unlikely that the Darcys' daughters would have fortunes of 30,000 pounds, but consider, if Darcy Sr. had had more than one daughter, they wouldn't have had thirty thousands a piece either, unless it was saved out of the family income. You're talking in terms of needing 20,000 to 30,000 per child, so one fortune of 30,000 wouldn't get that far once it's chopped up that many ways. I do believe that Darcy is the sort of responsible man who would not only spend less than he made but proactively invest and work to generate enough income to provide for his children, but even the daughters of the nobility rarely had fortunes larger than 20,000 or so.
In summary, I think Darcy would want to provide for any younger sons he has through lucrative careers by providing them with the education and opportunities needed, and that most of the cash would be saved for the daughters who, even if they didn't have fortunes as large as their aunt, would still be perfectly eligible.
As for dowries for daughters, those could be promised in future as well as present--this much money now, and additional money at regular intervals for a specified period of time, that sort of thing. I agree it's unlikely that the Darcys' daughters would have fortunes of 30,000 pounds, but consider, if Darcy Sr. had had more than one daughter, they wouldn't have had thirty thousands a piece either, unless it was saved out of the family income. You're talking in terms of needing 20,000 to 30,000 per child, so one fortune of 30,000 wouldn't get that far once it's chopped up that many ways. I do believe that Darcy is the sort of responsible man who would not only spend less than he made but proactively invest and work to generate enough income to provide for his children, but even the daughters of the nobility rarely had fortunes larger than 20,000 or so.
In summary, I think Darcy would want to provide for any younger sons he has through lucrative careers by providing them with the education and opportunities needed, and that most of the cash would be saved for the daughters who, even if they didn't have fortunes as large as their aunt, would still be perfectly eligible.