Nah I'm really not because supposing you get a discount for sending it in, you'll still be paying like £30+ for a preowned Wii U game, then whatever the discount price is. And before you counter that, I find it very unlikely Nintendo will give it you for free for sending a game to them. That will just cost them more money to process all these games being sent to them while making no money back in return. Just no way.
Don't see them ever running such a thing anyway. Nintendo will happily charge again I think. More likely any ports will just be a bit cheaper than new games. Or just the same price, because you know, 'screw them'