Even if that red demon saint left behind some bud from which to recreate himself later, he will still be absent for a long time. But of course it would be worth it for the demon side nonetheless, compared to letting Bright Moon escape. That being said, it's the demon domain, so how would the red demon saint's own continents persevere without an active demon saint? Logically all the neighbours would conquer the place and divide the continents among themselves. They are bloodthirsty demons, after all. Upon waking up, the red demon saint would find out he's a guest in his own palace.