I'd concur with the consensus that Brook's King Lear is a nihilistic production, being influenced by Jan Kott's essay "King Lear or Endgame", which states;
It makes a tragic mockery of all eschatologies, of the heaven promised on Earth, and the heaven promised after death... orders of established values disintegrate. All that remains at the end of the gigantic pantomine is the earth, empty and bleeding.
The above statement is reflected in the end of Brook's adaption, particularly in Edgar's final speech being 'relocated' so that it is said whilst Lear is still alive (Quite sad in itself, being abandoned before death), and the shot of Lear falling, the camera lingering upon the sky for a few moments before the film concludes. Of course, this perspective is reflected throughout the entire 'text', from the opening shot of a 'spellbound' populace onwards..