Kaido was keeping wano in order in other words yonko system was intact . After kaido is done its chaos in the system which navy truly despise.
Greenbull prefers old order when kaido was around
It's not different than the real world. Take Iraq under Saddam Hussein or Libya under Muammar Gaddafi as examples.
- Some people thrived, others suffered
- The countries were very stable, authoritative states
- There was one clear cut leader
- The world at large was stable
With Hussein/Gaddafi killed due to possessing significant resources and not paying enough of their heavenly tribute to the World Government (USA/Nato) Iraq/Libya were destroyed as well as the power balance in the world. Migration, famine, drought.
Others, like Putin, are yonko that even the WG haven't dared to touch (until now).
Yet others like Kim Jong Un have ancient weapons (nuclear) that keeps the WG from invading.