What do videos 'prove'? I mean exactly?
Let me explain it another way. You have X number of years left to you. You have Y number of hours you can devote per week to MA training. The numbers may differ for all of us, but the math is the same. So you take Y times 52 (assuming no vacations) and multiply that times the years you have left. That's how many hours you have to train.
You can spend 100% of those hours training in one style.
You can spend 50% of those hours in training in each of two styles.
And so on.
With each additional style, you decrease the amount of time you have to continue training in one style.
That's math. You can like that or not like that, you can't change it.
The assumption appears to be that one can train 'enough' in a given style that additional training would not make a significant difference.
That is where we apparently disagree. I feel that there is no end to the path, no perfection to be had, no end-of-the-road for training. You will not get to the point where you have learned all there is to learn. So any time you take away from training is time you do not have to get further down that road.
"Very good at both" means nothing to me. What is "very good at both?" It's purely subjective, as I said before. You think you can point to this guy or that guy and say their videos 'prove' they are "very good." I say that they'd be better at one style if they only training in that style than if they divided their time.
In any case, this discussion seems to come up a lot. Let me be very blunt about it. New people show up on MT all the time and ask for advice in training in X, Y, and Z, styles. Which ones should they choose? Which blend is the 'best'? And blah blah blah.
You know what? Everybody chips in with their favorite arts like it was a damned coffee shop, and who cares. What I think is that the new people who ask these questions are not going to train at all, let alone in multiple styles. They aren't serious, they're tire-kickers. They're looky-loos. They won't spend more than a week inside a real martial arts studio in their lives. The same goes for the endless supply of people asking about learning from books or videos. Not only does it not work, they won't do it anyway, so what is the point exactly? They are lazy bums who won't ever get off their fat arses and train in any case. Wannabes and never-wases.
Bottom line is this, though. OP asked for opinions. This is my opinion; train in one style or suck at two of them. You don't like that opinion, that's fine. I'm not trying to change anyone's mind. I am offering my opinion.