What would have happened if the Bennett daughters did not find husbands?

If the Bennett daughters had not found husbands, their future would have been very uncertain, and there are a couple of reasons for this. Firstly, Longbourn Estate, from which the Bennett family derives its wealth, is entailed. This means that it can only be inherited by a male member of the family. Since Mr. Bennett had no sons, the next in line to inherit Longbourn (and its wealth) was Mr. Collins. Upon Mr. Bennett's death, if the daughters had not found husbands, there was a very real chance that they would have been made homeless; they would have had no legal right to live in Longbourn.
Secondly, the daughters needed to find husbands to ensure a good standard of living. Although each of the Bennett daughters has an inheritance to live on, you should have noticed that it is a very small sum. Elizabeth, for example, would only have £40 per year. If you compare this with Darcy's £10,000 a year, you can see just how tiny an amount this is.
The Bennett daughters, therefore, need husbands to guarantee them a place to live and enough money to live on. This explains Mrs. Bennett's frantic efforts to find them each a husband.