Keep in mind, it also very clearly serves the CIA's interests to suggest that Assange is compromised, so people should take those who push this idea with perhaps greater skepticism than they do Assange himself.
Also, control of a private key only demonstrates control of the machine on which that key exists, so it is not definitive proof of anything. It is possible Assange's machine was wiped or hacked, or that the person in control of that machine was killed or taken out of play. There have been multiple mysterious deaths associated with Wikileaks in the past couple years. It is conceivable that Assange never was in control of the private key and that one of those people was, in which case it makes perfect sense that he can make multiple convincing videos but not produce that specific key.
Finally, the question of whether Wikileaks is "neutral" (whatever that is supposed to mean) is not particularly relevant given their implacable record of truth in reporting. The leaks are most likely completely true, and as new information, should be evaluated independently/objectively.
As someone who has read a few histories of the CIA, I can say that anyone who is their enemy is my friend. Their worldview and approach to international action has caused mass human suffering on the world stage for decades, usually in the name of protecting things like unfair trade agreements, destroying unionization efforts & democracy, and eliminating political affiliations that are not in the perceived interests (in terms of world hegemony) of the US government. In popular media this is portrayed as a "necessary evil", but in reality it is a sociopathic service to power that degrades the soul of all humanity.
Syria is probably the most recent example. The CIA was envisioning an eerily similar scenario to the one that is playing out right now...in 1986.  I encourage you to read up on their origins and history, it is a fascinating if extremely unsettling story.
 - https://www.cia.gov/library/readingroom/docs/CIA-RDP86T01017...