Corbin was a 3 WAR pitcher in 2017 and a 6.3 WAR pitcher in 2018 in a career season. It's perfectly fine to believe that he's a 3.5 WAR pitcher going forward with potentially more upside. In fact, Steamer projects his worth to be 3.5 WAR on the nose in 2019. That makes him worth about $28 million a year on the open market. So, so far this contract looks OK.
Let's project this out a little further. As Corbin is going into his age-30 season, we can try and project his future WAR along an aging curve, where he loses roughly 0.3 WAR per season. Then, we have these numbers:
2019: 3.5 WAR
2020: 3.2 WAR
2021: 2.9 WAR
2022: 2.6 WAR
2023: 2.3 WAR
2024: 2.0 WAR
It's not unreasonable to expect that Corbin, barring serious injury, could produce a total of 16.5 WAR over 6 seasons. That's roughly 2.75 WAR a season, which would be worth about $22 million a season if we value a 1 WAR performer at $8 million.
It was thought that Corbin would recieve a contract in the area of 6 years and $126 million or $21 million a season. Left-handed pitchers do tend to age more gracefully than right-handed pitchers, it seems. There's no saying that Corbin won't fall off a cliff. But, that's true of any pitcher. Personally, I'm hesitant to give any player over 30 a six-year guarantee. That guaranteed sixth year came with the cost of deferred money. I think that the Nationals pretty much paid market rate here.
That being said, it's not a bad move for the Nationals. Corbin will be 35 when this contract expires, and if he's as good as those basic projections suggest, this is going to be a good deal. Of course, he may be better than 3.5 WAR in 2019. But, in any case, this is a good deal on the surface. If he keeps up the gains in his strikeout rate from 2018, this guy is probably a really good bet to pair with Max Scherzer and Stephen Strasburg as the best 1-2-3 starter punch in the game.