The stadium section is being done away with because its not due to attendance figures, you can get up to 12-17% more seats in that config, as well as the high ticket price areana getting new stands, meaning more money for the BRDC in the long term.
Also it could be to get the lap time up to arround the 1:35.000 mark, as current development on cars means that the race will be just under the 1 hour 20 mark, with 1:35, the race could be arround the 1 hour 38 mark. More racing for the spectator.
So its more Win-Win for all.