I read yesterday that Smith isn't very enthusiastic to throw away 20 years of seniority for 2 years as SoD.
Plus, relative to most Congressmen, Smith is a young guy in a safe seat (iirc, he's only like 49 and is already ranking member on the House Armed Services committee), he will probably still be in office whenever the Dems eventually retake the House which means he'll likely be Chairman one day if he's patient. Why throw that away to be a two-term SoD who will probably be largely shut out of the decision making process like Hagel was? I'd be shocked if Smith accepted the post in the event it was even offered, I suspect he's going to be a Congressman for quite some time unless he's eventually redistricted into a Republican district (which seems unlikely barring a major realignment, plus he's a pretty strong incumbent so I think he'd hang on even if his district was somehow drawn to be more competitive in 20-30 years. I doubt he'll be discomforted much in the next round of redistricting, even if the Republicans get another good map.
Note: Sorry for the length, I didn't mean to write a wall of text on Adam Smith's political future.