Millennials' drinking habits are causing a crisis for America's most iconic beer brands — and now...

I think they are failing because the product they produce is no longer what Americans want. Yes you will always have the die hard fans, but by in large Customers are leaving for a better tasting, quality, locally made Beer.
The numbers don't lie:
Overall U.S. beer volume sales were down 1% in 2017, whereas craft brewer sales continued to grow at a rate of 5% by volume, reaching 12.7% of the U.S. beer market by volume. Craft production grew the most for microbreweries.
Retail dollar sales of craft increased 8%, up to $26.0 billion, and now account for more than 23% of the $111.4 billion U.S. beer market.
https://www.brewersassociation.org/statistics/national-beer-sales-production-data/

Interesting that they don't mention craft breweries as part of the problem. Could also be folks that like better beer!