According to little fatty, after the battle at Dongling Valley three years ago, monkey fell into a frenzy and almost went insane, nearing causing a huge disaster. Thankfully, he was protected by senior crane.
However, monkey was disheartened and chose to leave Ethereal Peak with the spirit tiger following suit.
During that period of time, little crane, monkey and the spirit tiger had an extremely good relationship. If it wasn't for senior crane stepping in, little crane would have left with them as well.
To Su Zimo, it was not necessarily a bad thing that monkey left Ethereal Peak.
It had an arrogant personality and did not like to be restricted. Without him around, it was bound to cause trouble and it was better off returning to the barren mountains.
Given monkey's abilities, coupled with the fact that the spirit tiger was with it, they should not be in any danger.