Though the problem is real, I don't think it's important to clarify thinking about what constitutes a great brand, because then one can understand better what exactly is fading away.

To be the supporting pillar of a business with unusually high profitability, a great brand doesn't come from brand awareness, or attitude towards the brand, or even dominant market share. It comes from the answer to the question: does the prospective customer, rightly or wrongly, perceive that there is no close substitute? A "yes" answer to that question is the only thing that gets you an economic moat.

So, I think the key thing in your question post is that private label products have improved considerably. Previously, folks didn't consider them to be viable alternatives to a known product, but now they do. This has happened to so many product categories that the traditional deep skepticism of a house brand has eroded.

The brands that will continue to survive (in the sense of supporting high profitability) are the ones still perceived as unique: having no close substitute. The best way to do that is to BE unique (including things like being mandatory due to regulations). Though of that way, it's easy to see why branded foods have suffered the most: there are no trademarks on recipes, so it's almost impossible to be truly unique if a competitor wants to make the same thing. It must have been great for Wrigley when owned the output of essentially all the world's chicle plantations.

There are other ways to be perceived as "no close substitute", but they are so rare that it's hard to analyze what makes them tick. A Coke drinker won't buy a Pepsi that's 10 cents cheaper, even if they can't tell the difference in a blind test, but that's such an extreme example it probably doesn't hold much utility for other brands. One of my in-laws has a tattoo of the Gitanes cigarette logo, go figure. From Wikipedia: In July 2016, the French government considered a ban on both the Gitanes and Gauloises cigarette brands because they were deemed "too stylish and cool".

Jim