I agree. It ought to be similar to how the mystery of Donna is resolved about two-third of the way into "The Runaway Bride"-- but she continues to play an active role in the story. I think it's especially crummy that Lake can't even be a hero to save his own son!RTD puts a really interesting piece on the storytelling board -- Jackson Lake. And it's that piece that really drives the first half of the episode. But once the Jackson Lake mystery is resolved, the energy of the episode dissipates as it turns into the Doctor battling steampunk Cybermen, and it's not very interesting.
Davies really needed another draft, because I think "TND" would have been easily fixable. Have Lake play a role in the climax, rather than standing around as a passive observer. Have Lake, not the Doctor, save his son. Have Lake discover that, while he's not the Doctor, he can still be a hero.
And finally, put Jackson Lake in the damn balloon.
Unfortunately, in this case there just wasn't time. With "The Next Doctor" filmed at the end of Series Four, Rusty needed to squeeze out that script as fast as possible. As it was, they had to delay production for a week so he could have a chance to write it. The fact that it fell at the end of an intense season probably didn't help, either.The first half is wonderful. The last half is terrible. Davies needed another draft, though he also needed someone far enough removed from the story to tell him why the script didn't work.