Both Sovet style and Cuban boxing tend to implement high guard as the go-to basic defense, though I agree, it's a part of the flow of their defense, rather than singular focus like it is for the likes of Winky Wright. It's extremely easy to learn how to shell up into the high-guard, just how it's easy to teach a man very basic crossguard position, the shell or the lock, as Archie Moore called it. Thing is, there is also the head movement and especially upper body movement, footwork, extended checking hand, and most importantly the give-and-take concept of playing with expectations of being hittable versus suddenly becoming slick and countering. Canelo and Gonzales really are the two great case studies of how complex of a system high guard can be. Blocking is not optimal, yes, but the best of pros learn to disperse energy of incoming blows with miniscule movements. They rarely take the blow passively like a punching bag. What's definitely necessary for the high guard, is truly assertive mindset. Understanding that by shelling up in such way you are vulnerable to control game and hand traps, but at the same time provide no easy way to hit you, meaning you can occupy and control ring geometry in a much more oppressive manner.
Very good post. I agree with it, but You put it better than I could. I'll add one thing that I heard Mateusz Masternak talk about. He wasn't ever typical "high-guard" fighter, but when He signed with Sauerland, He trained with Ulli Wegner for couple years and Ulli was training fighters in that kind of style and technique for decades. One thing that Masternak talked about is importance of proper breathing while staying behind a guard, which gives You ability to be relaxed and rest in defensive position.
There's an art to being able to take shots with a high guard. Clottey and Wright were both masters. Floyd also had a terrific high guard which he displayed for rounds against Mosley--probably one of the most excellent displays of the high guard I've ever seen. The main problem with a high guard, which I found out about in my first real spar, is that taking shots on your gloves isn't a piece of cake and you can get hurt or even staggered from a hard blow that you absorb incorrectly through the gloves. It's better to not be there for the impact of the punch, rather than trying to take the impact. The high guard also means that you have to throw your punches from a position that is too high for optimal punching and from a more difficult angle with fewer options. Punches flow best from about shoulder level and it's easier to engage more of your body than throwing from your forehead. I think every fighter develops a decent high guard and normally will deploy it in emergency situations or as a second style, it's one of the very first defensive tactics anyone will learn--have a stout and sturdy guard. It's like a basic layup in basketball, you'll learn it and practice it and generally move on from using it all the time in favor of more professional options. When **** hits the fan, the high guard comes out a lot. It's usually just a piece of strategy rather than the whole gameplan, but it is obviously a very effective way of using your defense.
Usyk is more of a technical fighter where as Leonard and Ali were more athletic fighters that relied on their speed and reflexes. And Usyk is an exception there isn't many fighters in history, who had great footwork moving all round the ring that employed a high guard style. Some fighters do better fighting with a lower guard being more unorthodox, and if you tried to make them more technical it could actually make them worse fighters overall.
Dmitry Bivol uses the high guard defense really well. Of course it helps that he also uses great footwork as another means of defense.
Plenty of Eastern Europeans use the high guard well. Usyk, Beterbiev and Bivol all have excellent high guards. People like to trash Beterbiev's defense. But they negletc to notice how his chin is tucked and his hands are up. Just basic stuff, but a lot of fighters neglect fundamentals.
Usyks barely uses the high guard. Benavidez relies much more on the high guard, he is constantly sitting behind it, as its fundamental to his whole fighting game.