That's much more than a manager-level decision. But also, relievers just break. I mean: they broke Robertson and Neshek and Hunter and I think even Nicasio too. Giles has broken many times since he left us.
Beyond that the issue was more COVID, and Seranthony's personal situation. That cost him an entire year.
The only inviolable rule of relief pitching, I think, is that homegrowing a Giles or Seranthony (or Neris, yes!) is truly the only foolproof thing (but even that is subject to injury, and another reason why Hector was so underrated). All of the other ways: big $, medium $, dumpster dives, fungibles really do feel more like roulette. But the Phillies also had some especially bad spins these last five years. Heck, Robertson was specifically signed for his durability!