Here's why I'm not worried about them signing a SP:
Eflin, Pivetta and Velasquez have a good shot of pitching much better next year behind an improved defense.
All three were better than average SPs by advanced metrics, and with a year under their belt, and top notch FBs, they're more likely to progress than regress. Add them to Nola and Arrieta and this should be one of the better pitching staffs in baseball. Throw in Eickhoff, De Los Santos, Anderson, Irvin, Suarez, Romero and Medina as minor league depth and they have some injury protection. And if the defense allows SPs to go an extra 1/3 to 2/3 of a inning per start, that's 50-100 less IP for the bullpen next year.
So yeah, adding Harper or Machado would help, but if they add 2 WAR with a second tier OF instead of 5 WAR with Harper (v 0 WAR with Altherr/Williams) they're still going to be improved over last season. Machado at 3B would be a bigger lift, since his defense would be far superior to Franco, Machado at SS probably a gross overpay.