The line blurs somewhat between "playable" and "popular" when it comes to tournament results. Is Magus of the Future Vintage playable and somewhat unpopular? Is Notion Thief popular but not really playable (using this as an example as I am well aware of Tom Dixon's dislike of this card)?
Popularity and playability are related though. A card's playability may hinge on another card's popularity.
I think Notion Thief and Magus of the Future are playable. Notion Thief + Dack Fayden is a two-card combo that basically ends the game.
I had a hilarious game at the Waterbury where I cast Show and Tell and dropped Bargain, by Jeremey Beaver dropped NOtion Thief. I lost the game right there.
I also don't think that omitting Black Lotus or another card is "proving a negative". You generally start by playing Black Lotus and finding that the card does not perform as well as you hoped and that it's not worth the opportunity cost. The tricky thing with Black Lotus is how ubiquitous it is and how powerful the effect *can* be. I'm trusting in Will's, Forino's, and Nick's experience with the card and the deck, which I value over Tom Martell's at this point. I'm not sure this is "empirical", though I definitely have "experienced" being thrashed by all three of them.
And I would agree that all the cards mentioned are playable. Really, Darksteel Juggernaut is tough for red to beat.
Regarding Black Lotus, I wish someone could find the big thread where Nick and I went back and forth, but I do think, taking the other side, that players who've played hundreds of games with a deck can perceive costs or value differences that aren't abstractly visible. Case in point: the landstill players who cut Time Walk (because a cantrip isn't as valuable as something else in that slot), or, when I was playing mono blue control back in 2004 Vintage Champs Top 8, where I didn't play Brainstorm, but played Impulse because the few times where I didn't have a shuffle effect were unacceptable costs, and all I wanted to was to find the one card I needed.