That is very impressive work.
I don’t like effects for effects sake. In other words I hate how people insist on jazzing up reality. However I am interested in ways of making transitions more natural, and less abrupt, i.e. less noticeable!
Your technique could be better than a fade transition if done subtly. It’s a pity it can’t be easily automated as a standard transition but it’s very clever and clearly presented in your tutorial.
In the tutorial I made the transition 3 seconds long so viewers could better follow how the fade progresses.
I agree with you, in transitions less is more.
I usually use this transition in 2 different ways; a 1 second or less subtle transition as you mentioned and a 3-4 seconds transition where the subject in the second video is masked in first, kind of what I did with the Statue of Liberty in the tutorial.
In some NLEs fade transitions have attributes and settings, with Luma Fade being one of the options. I agree, it would be great to have Luma Fade as a stock transition.
I also would find value in having the ability to keyframe the value of the keys in Luma Key and Chromakey.