Arrieta is not closing that well, but he has performed as advertised, basically being a 3 WAR player. Since wins are worth at least $8 or $9 million he has been worth his contract. His velocity has been fine also, so he does not seem too much of a risk to regress absent injury.
If this team had to find 180 more innings from other pitchers (Lively, Leiter, Eshelman, Irvin, Suarez, De Los Santos) the results might have been pretty ugly.