it sucks that they didn't have more foresight. I imagine those adapters are not that hard/expensive to make, they should've just made a billion of them. I mean, those things were sold out across the nation on day 1 it seems.
Edit: Welp, after looking on the back of the box I see that the Wii Classic controllers are compatible with Wii U Smash, so I don't have to buy anything as I already have 4 of those!