It's the same problem with Silence of the Lambs; as groundbreaking as it was a the time, 1,500 serial killer films later, it's hard to see the original without being aware of how its themes have been pounded into dust by the Hollywood grist mill.
You're right, of course, that Holy Grail has been over-quoted and over-referenced for thirty-odd years. But that doesn't negate the fact that the film was a landmark in surreal humor.
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