FG accuracy really falls off a cliff when the LOS is outside the 35. At very best, SEA could hope for a 20% shot from 61 yds, and that's very generous, allowing for solid footing and good weather.
We can typically expect 4th and 8 conversion attempts to be successful 37% of the time in that part of the field. SEA had a TO remaining, which means they could use the middle of the field. ATL would have to guard deep against a game-fatal TD, so I would put their chances at 37% at a minimum. A successful conversion means the LOS would be at the 35 at least, making the kick a 50/50 proposition at least.
In total, the pass-then-kick strategy would be worth:
0.37 * 0.50 = 0.19 WP at least.
Meanwhile, the kick right now from the 43 would be worth:
0.20 WP at most.
Obviously, SEA should have passed because they missed the kick.
Just kidding. You could make a case either way. The local conditions--kicker pre-game distance, wind, the fact that Tavaris Jackson is your QB--would all come into play. Personally, I would have attempted a conversion then kicked, given how generous my assumptions were in favor of the kick-now strategy.