I doubt McCarthy stepping down as story editor had any impact on them leaving. Amy Keating Rogers notably did say she left because she wanted to work at Disney on Star Darlings and MA Larson was writing a book at the time with Pennyroyal Academy. And Corey Powell was already gone after season 4, before McCarthy stepped down at the end of season 5. Plus, Dave Polsky did stay a bit longer after McCarthy stopped story editing, writing in season 6 and he did say he only left because he was showrunning Hanazuki at that point. So I doubt the writers leaving were impacted by McCarthy not being story editor.
So how'd Lewis/Songco fare as story editors in season 7?
Overall thread:
Wasn't McCarthy still a writer on several episodes of season 6 with Flutter Brutter, The Fault In Our Cutie Marks, 28 Pranks Later, Where the Apple Lies, and Top Bolt?
Also, it's not like writers leaving a show after a certain point is uncommon, especially for long runners. Remember SpongeBob, Fairly OddParents, Rugrats, Adventure Time, and Regular Show with how many writers left there too?