What happens after total core kill:
* No more income from trade. You still get income from Population and Infrastructure from all of your colonies.
* The mandatory pirate bases will always target your worlds for -3/-50% activity because your worlds are all that is left.
* Killing said pirate bases respawn almost immediately after elimination, meaning pirate activity is back on your colonies.
If you want to make a lot of money after you kill all of the core worlds, you need to colonize a bunch more worlds, and doing that requires alpha cores, which means farming Ordos in red systems. With seven worlds and max colony skills, income without trade is about 10% of income with trade.
If I want to colonize the entire sector, total core kill is only the beginning. Grinding Ordos after Ordos for cores until all worlds are colonized is the marathon grinding part.