It is very interesting to note that both John and his wife are happy to dispose off Mrs. Dashwood within the next few years.
"To be sure," said she, "it is better than parting with fifteen hundred pounds at once. But then if Mrs. Dashwood should live fifteen years, we shall be completely taken in."
"Fifteen years! my dear Fanny; her life cannot be worth half that purchase." Ch. 2
Even when Fanny is being pessimistic about her mother-in-law's life-expectancy, the longest extension she can imagine is 15 years! I would have thought she could have made her case more effective by predicting Mrs. Dashwood would live for another 30 years. It must be a pleasure being them!