Fringe arms are fringe until they're not, that is, most solid starters spend their first few years as fringe starters until the light goes on (i.e. they fix mechanical issues, add a secondary pitch, develop movement on their FB, etc.). Eflin is on the verge of the next step, Pivetta and Velasquez are running out of time to make that jump.
There are few reliable FA SPs available at a reasonable price, two years ago it was Darvish and Arrieta, how's that working for you. Last year it was Corbin, so far so good, but 5 years left from age 31-35 - fine for a team with a window closing soon, but not for a team trying to become competitive.
Trading for a top pitcher is expensive, and requires a deep farm system that can afford to give up 2-3 top ten prospects. Phillies are years away from being in that situation, we gave up 2 of these guys and got two starters (Segura, Realmuto). Right now we don't have 10 guys who deserve to be considered top ten prospects.
Adding patchers on 1-2 year deals is a crap shoot, remember Morton? Right pitcher, wrong year.
But it's only money, we tried to build a bullpen that way, oops.
[between Arrieta and the veteran RPs, we're paying $61M for 1 WAR, Eflin (BR) is worth 1.5 WAR]