Last year wasn't his first very good year but it was his first elite year. Hes being paid based on that elite performance. Its very possible that hes figured somethign out and will be elite for the next 4-5 years. Its equally possible that the addition of the humidor impacted his performance, or there was some luck involved.
Its very possible that his increased usage of the slider has been key to his success and as long as he remains healthy he'll provide elite or close to elite performance but the slider will lead to an injury in the short to medium term.
I would probably have been excited if he signed for the Phillies, big FA signings are always exciting but I'm not overly disappointed to see him sign elsewhere.
FWIW Steamer projects him to have similar performance to Arrieta and Pivetta next year. Adding ~2.5-3 wins would be helpful but hed only be bumping a 1-2 win player out of the rotation and I think there are smarter ways to add 1-2 wins to the rotation.
I like Keuchel and I think hed be valuable on a short term deal and potentially outperform or come close to matching Corbin over 2-3 years but the draft pick is off-putting.