Why can't they just get new people? No one knew who Tom Holland was before he was cast as Spider-Man. Not sure why they've become so obsessed with big names. That was never what made the MCU great.
Thing is, no matter what they did, it would've likely been a weak and highly criticized transition. I can see why they wanted to at least try to continue on with the character even if it was half assed in this scenario.
This isn't on James Gunn. The CW is under new ownership and they basically want to lean more into cheaply made content and S&L is more on the expensive side.