I agree that if the estate had been left to Henry Dashwood outright, he might have done something to provide for his daughters more substantially. As Jane Austen writes, " it was secured, in such a way, as to leave to himself no power of providing for those who were most dear to him, and who most needed a provision, by any charge on the estate, or by any sale of its valuanble woods."

Jane Austen tells us what his intention would have been as far as his wife and daughters went. They would draw an income from the estate or he would sell part of it off and leave the proceeds of that to them. There was never any question of not leaving Norland to John, it's just that he might not have left the estate whole and unencumbered to John. This is what the old uncle's will prevented, and probably old Mr. Dashwood wanted to be sure that wouldn't happen. It would have been unheard of to leave an estate to the eldest daughter in trust for a son who may not ever even exist, when there was a male heir with a male heir of his own--no matter how much Henry Dashwood loved his daughters.