Ava was not the fairy from the prophecy, even though it was not yet understood she might not even be one. Yes, she does have qualities as a fairy and it was something they were yet to grasp completely. But it still remains that the master's search will continue.
This was solely the reason Severus asked his master such a thing even though he knew he shouldn't bring it up, but he knew his master had been thinking about it ever since they found out the truth about Ava.
William remained silent not having it in him to answer, he knew Severus was bound to ask such a question of him, there was no way to escape from it. In fact, William has been pondering this too.
Does his search have to continue?
Does he have to go another search for centuries?
For the very first time, William had not the faintest idea of what to do, but all he cared about right now, and all he was truly centered on was Ava.
It was all about her