I think I lament more the loss of decent versions of beers, some of which are still about in bowdlerised form, than beers from long dead breweries.
Draught Bass is probably the prime example but Gales HSB in its pre -1980 form and Pedigree in its pre circa 2000 form would fit the bill. The inclusion of HSB pre Fullers reminds me that Boddingtons was dumbed down before they sold out to Whitbread. Ruddles County was an early example of a beer destroyed by big brewery accountants who fell for the myth that the brand was more important than the product. Jennings Cumberland seems to be another. Pub Curmudgeon mentioned Gales 5X, which he didn't know was actually a blend of BBB and Prize Old Ale, gorgeous stuff it was too; I miss that as much as Prize Old Ale, decent White Shield (although it's not bad these days) and Courage Imperial Russian Stout.