If his plan to hide his phylactery in his totally insane Astral Plane Fortress worked, he'd probably have been as invincible as it would ever get and the point of fighting him at all would have been gone.
For all the ill Redcloak did in creating the abomination "Xykon" the world (the entire multiverse) probably has to thank him he put a stop to that last escalation.
An excellent point.
Spoiler: My current mood
The world is grey, the mountains old,
The forge's fire is ashen-cold;
No harp is wrung, no hammer falls:
The darkness dwells in Durin's halls;
The shadow lies upon his tomb
In Moria, in Khazad-dûm.