"A way out from here," William knew by telling them this, the secret wouldn't be kept within this group and would spread. Yet he didn't care about any of that.
This world had tons of formidable masters. But without knowing of a certain method to go out, no one would keep aiming to get stronger anymore, nor they'd be concerned by the issues of this world.
The delight of crushing obstacles and jumping from one grade of power to another was indeed priceless. Yet if the end result was for no one to go out from here, just live a little longer, a dull form of life that didn't suit a true master, then depression was the sole result of such a scenario.
William wanted to help the masters of this world, telling them that there was a way out from here. Not to mention his desire to lure this group of formidable masters to come and follow him back to his tiny world, follow him later on to the outer world.