A friend of mine who worked for Diageo, in their Guiness division, said the brewery's lease entitles them to obtain water from a stream that comes from the Liffy.I was intrigued when on a brewery tour some years back to find that (some) brewers strip the water they use (for certain brews) back to (best described as neutral) then build the water profiles back up to reflect the water from the area of the original brew (pils etc).
Example for Guinness
View attachment 87175
Brewing Water Target Profiles - Brewer's Friend
www.brewersfriend.com
Basically, he said they haven't paid for water for hundreds of years now and legally - can't confirm it's entirely true but I can't see him making it up.
Another point, more similar to your own above, is how Diageo can replicate the taste of most whiskies in a fraction of the time it takes in barrels etc.
It would be much cheaper too and only real connoisseurs can tell the difference, but they'd have to charge much less: it pays for them to actually be traditional.