Did you see the chart on MMO-C's front page the other day that shows that upwards of 60% of all players had completed many of the MoP raids up through a final raid boss? Do you imagine for a minute that most of that was anything but LFR given historical participation rates. Flex will reduce that some but it's still likely to be far and away the most popular raid mode in the game for the foreseeable future. Blizzard would only shoot themselves in the foot by doing anything at all to make it less popular.