I'll be honest, I don't remember Allanon ever explicitly telling Wil he planned to make him a Druid, but for sake of argument I'll assume you're right and I just missed it. If so, I would have to chalk it up to some sloppy writing by the people who adapted Elfstones of Shannara for TV, because that plot element isn't anywhere in the source material, and it doesn't fit Allanon's actions throughout the season.
As a Druid, Allanon always knows it's going to be his destiny to face the Dagda Mor. The reason he seeks out Wil is because he knows Wil will have inherited the magic of the Elfstones from Shea. (We hear almost nothing about Shea Ohmsford in the show, but he's the main protagonist of the first novel, Sword of Shannara, so perhaps we'll get more about him in future seasons.) Allanon knows that he's going to need help to save the Elcrys, and he knows about the Ohmsfords and the Elfstones, so he decides to track down Wil and recruit him.
Apart from some brief training in using magic, the role Allanon seems to push Wil into isn't learning to be a Druid, it's protecting Amberle. If anything, Allanon seems to be focusing on Bandon as the next Druid, not Wil. He never gives Wil any kind of combat training, or a weapon to use, and doesn't send him out to the Demon's henge to fight. In fact he seems to send Wil as far from the fighting as possible.
So, I don't think Allanon ever planned to make Wil a Druid or expected Wil to defeat the Dagda Mor. I think he hoped Wil's magic and the Elfstones would tip the balance in his favor against the Demon, but Wil's purpose was always to make sure the Elcrys seed got where it needed to be.