I think that 5-year olds had a lock on 5Y's well before it became a 'named problem solving tool'. How many times have our kids at that age asked 'Why' about everything?
How many 'Why's' do we answer before we say, 'I don't know, go ask your mother or father!'
Again (to me), the traditional 5-Y tool is technically incapable of expressing multiple paths of logic that occurred simultaneously. It treats failure like it always happens in a linear pattern (never multiple, simultaneous paths).
Like many other veteran analysts, I do not believe there is a magic number of '5' levels deep for our analysis. Why not '3' or '8' levels?
5-Y's also gives the impression there is only 1 root cause. Having been doing this for nearly 30 years, I have never been involved in an investigation that had only 1 root cause. I have never seen a professional investigation of a high visibility event in any industry with one root cause.
We drill down until we understand the flawed reasoning from well-intended decision makers (we call these Latent Root Causes or the flawed management/organizational systems).
I do NOT view 5-Y's as a valid and capable RCA tool for higher severity/priority events that require more comprehensiveness and depth. As always, I am open to debate on this (and I am sure I will hear from opponents), but I will continue to ask for case studies where traditional 5-Y's was used successfully on such high severity/priority events. Perhaps such a case will enlighten us '5-Y' skeptics, as it being a valid RCA tool.
A big part of the problem with these types of discussions on the various forums is, there is no universally accepted and shared RCA dictionary of terms; we all define things in our own way. I think many of us are saying the same thing, but the lack of uniformity of terms makes it appear we are in conflict. We all have unity in purpose though!