Nothing that should surprise most people here, Arrieta is no longer an elite starter, and the Phillies aren't paying him as one, Nola is the better pitcher at this point, and Pivetta may soon surpass him.
But his contract only requires him to put up 9-10 WAR over three years, which is basically a #2/#3 starter, which he still is and should be for another year or two. His velocity is actually up a notch over last season.
Arrieta has good stuff, but average command, his movement makes hard contact more difficult, and he's pitching more to contact. But he has four solid pitches and his 2 seamer has great movement. If he finished the year as a 3.50 ERA/FIP starter with 30 starts and 180+ innings and 3-4 WAR, he's earned his money.
As long as you accept Arrieta for what he is, solid veteran pitching depth, it was a fine signing.
He'll buy time for De Los Santos, Irvin, Romero, Kilome, Sanchez, Medina et al to develop.
If you fooled yourself into thinking we added an elite starter, well, prepare to be disappointed.