The problem was that sales of the more bitter bottled stuff kept dropping.  Consumer tastes change and Guinness had to adapt somewhat to survive as a mainstream brand. But you are right that when Guinness became Diageo it became a marketing rather than a production company.  The corporate motto was "a drink for every occasion", and to that end a number of champagne, wine and spirits brands (and companies) were purchased to fill out the portfolio.  But they never purchased a major lager brand to replace Harp which indicated that they didn't really see beer as part of their core business.  

I once asked the Global Corporate MD whether they were going to buy a lager brand.  His response was that they might buy a lager brand if they didn't have to buy the Breweries as well. The Marketing suits in London didn't understand and didn't want the complications of actually running a brewery.  Guinness was strictly a legacy brand to be automated as much as possible.  Production was a non-core activity to be outsourced wherever possible. Diageo is a brand management business.

Index of Frank's Diaries