The Sun King still didn't want to leave the South Pole and wouldn't leave it to join the fight if he had a choice. He also didn't trust the new gem kings completely, but he didn't have a choice but to appear and fight right now.
He didn't have a choice because the barrier shielding the South Pole had also reduced in strength. Just like him, the strength of the barrier had fallen from 100C to 95C.
This means that the barrier is no longer an impenetrable shield. If the strongest gem kings or the strongest invader want to kill him, they can now breach the barrier of the South Pole.
Secondly, the weakening of his strength and the barrier won't stop. The 5% weakness is just the start.
If nothing is done to stop the invaders from continuing to steal his power, his strength will continue to weaken. This means that he will only become easier to kill over time as long as the invaders aren't stopped.
