Or William could have just got lost in the bush, and has not been found. Eg: Fallen into a small space, curled up inside a rotted tree for warmth.
Even if William is found out there someday, at this late stage it will be one heck of an uphill battle to work out how he got there.
At the inquest SC Williams spoke of .... "“There is certain methodology to assist in understanding what they call ‘lost person behaviour’”
"“He was engaged in fantasy play [playing daddy tiger] as a child that took me in the path of did he wander off?"
"He said he thought of William’s ability to walk, climb, jump and run, and questioned whether there may have been an animal he could have followed, “which is typical of three-year-olds”.
William Tyrrell’s foster dad searched bushland for days
I am reminded of Daniel O'Keeffe (a grown man) not being found until 4 years after he went missing, because he had lodged himself into a small cavity in his parent's own home and wasn't found during the searches for him.
Link