Not to be too pedantic on Christmas Eve, but have you ever been part of a Root Cause Investigation? If there is no complicated equipment malfunction, there is no crash. That is the root cause. If these guys had been flying a DC-9 instead of a 737 Max, the folks would still be alive. The "root cause" is the complicated aircraft malfunction.
No, I have not ever been part of a "Root Cause Investigation".
While I'm not sure if an NTSB crash investigation qualifies as such, they do usually conclude with a "probable cause" finding.
Given that, I think I'll go with Mr. Feith (Investigator in Charge of the NTSB “Go-Team” from 1993 to 2001):
“The NTSC stated the pilots, especially the First Officer, had significant training deficiencies and lacked basic flying skills. These same deficiencies occurred during the accident flight. These two pilots had no business being in the cockpit and the airplane should not have been operated because of all the maintenance issues that began at the beginning of October, and were not corrected, making the airplane unairworthy.”
Given that, I'll agree with Mr. Feith. I think the primary causal factor is unqualified pilots.