With a family reputation, family history and legacy at stake, it was not about spoiling little Harry or leaving everything to him because he was cute or because he was fascinated him. Jane Austen was being ironic to suggest that the little boy's 'cunning tricks and a great deal of noise' etc. would be the main influence in such a momentous decision. The opening line "The family of Dashwood had long been settled in Sussex" shows just how important this was to the old uncle.

The thought of having that family legacy tied up and that respectability and long settlement on the estate continue for another three generations at least was irresistible and far outweighed anything he felt for his nieces, which is the whole point. Under the circumstances and at that time ( not looking at it through a 21st century perspective), settling an estate in consideration of one's fondness for a bunch of young female teenage relatives would be unheard of.

This mentality has a very strong grip on people. I know at least three families from the small town where I grew up (and one of these was a very close friend in high school), where the family ranch and land etc. was left to the only son and the daughters of the family got nothing. We might well cry that it is unfair and that it is not right--and in our time, most would agree--but that's not how it was 200 years ago. I think most people would have been astonished if he hadn't
left that large, long settled estate in the male line.