It depends on a lot of variables.
Having been on YouTube since 2012 and having two channels, I can tell you, luck does play a part but a very small one. So does advertising, branding, consistency, relevance, quality of content, content similarity, content frequency and so on. There are so many variables that making such a generalization is not really the way to go.
Yes self-criticism has a place, it too is a variable. Yes, the content you make is also a variable but simply chalking it down to content that 'isn't engaging' isn't really enough. There are many variables that affect the success of a video as well as many variables that affect viewer drop off and engagement.
Just because a channel may not get a consistently high frequency of views per video does not mean they are making bad content. Similarly, just because a channel doesn't get a consistently stable frequency of watch time does not mean they are making bad content.
An example of what I mean being that if a channel is mixed (my bigger gaming channel being an example) - it will have long videos of up to 20 minutes as well as short videos than barely scratch 2 minutes. On average, the 2 minute videos will drag down the watch time average percentile for my channel. This is simply because in comparison to other videos, it may not seem like it has a lot of watch time but in reality a viewer may have watched 70 or 80% of the video - it is just that video is short in duration.
In contrast, longer videos where people watch maybe half way through will affect the drop off duration - negatively - as the video duration is long despite the watch time being greater than that of shorter duration videos.
YouTube has so many variables that affect the success of a channel and its content that saying 'you won't succeed because of X, Y and Z reasons' is somewhat ignorant. No matter who you are or how much experience you have. Similarly, there is no one formula for success and saying 'you will succeed if you do X, Y and Z' is equally ignorant.
Finding a way that suits you and your channel (as well as your subscribers - no matter how many) is all that really matters in the end.
Whoever this French streamer is, he has a point that content is the most important variable but what he may (or may not) have missed is that it is a far cry from the only important variable.