By the earliest accounts, McGinness (and the original "kids") gave up after digging 30 feet. It wasn't until the Onslow group that they excavated down to 90 feet.
I would accept that the original pit was a natural sinkhole except for the coconut fiber evidence. As we all know, palm trees don't grow in that region. Coconut fiber, called coir, was a common trade item because it was used like hemp. Common applications were rope, mattress stuffing, and matting. It can also be mixed with pitch to make a substitute for oakum - a common material used to waterproof ships hulls.
I also find their "discoveries" more interesting than a treasure that I find improbable. If they actually find solid evidence that the Money Pit is a human construction, it begs the question of why pre-industrial people would spend the resources to create it.