Let's do a brief summary of what they accomplished with a plan that almost all of us already considered dumb last week.
They have been very lucky and (with two deus ex machina) they "just" gave a dragon to the NK.
In the most probable case they would have also lost Jon Snow. Can you imagine someone who goes to Sansa and say:
"we lost Jon Snow, the King of the North"
"did Daenerys kill him?"
"No, he went beyond the wall to capture a wight" (???!)
On the other side, the positive thin