As he moves past prime, I don't expect improvement. True, the 5.50 ERA in 2019 might not be his new norm, but the 4.65 ERA in 2017 and 4.92 in 2015 are also clues that he is in decline.
Even his even years have shown decline from 3.15 in 2016 to 4.23 in 2018. His WHIP has been over 1.36 in 3 of his last 5 seasons- the odd years.
I don't usually hold one bad year against anyone, so you are right to call me out on that, but the age thing worries me.