Are the Teachings of Trout and Ries Relevant in 2019?

For decades, marketers have considered the wisdom of Al Ries and Jack Trout to be the gospel of positioning.  In their two books, “Positioning: The Battle for Your Mind,” and “Marketing Warfare,” Trout and Ries explained their concept that:
Consumers only have space in their brains for a handful of brands in any one category – say fast food, soft drinks, airlines, or athletic shoes.
The consensus number-one brand is the “defender.” Their job is to stay on top and never even acknowledge that the lesser brands exist. Example: Coca-Cola
The consensus number-two brand is on the “offense.” Their job is to try to knock off number-one with comparisons that make their brand look good. Example: Pepsi and the Pepsi Challenge
The next-level brand or brands are called “flankers.” They are not strong enough to take on those top two brands, so they carve out their own position in the marketplace. Example: Seven-up – The Un-Cola. 
But in today’s world of digital disruption, some small, start-up br...