There is no proof of that because these numbers are meaningless and purely arguing materials.
Even in the most exclusive time of WoW, there was like 2-3 % bleeding edge (and I say "bleeding edge", not just "hardcore") guildes overall - the ones that killed Illidan when most of the people were still wiping on Kara/SSC and the like. There were about 10-15 % more guilds that you could consider "hardcore", or at least "semi-hardcore". And there was a lot of more casual guilds (all this from the numbers we saw on progression sites that scanned the armoury to see the proportion of people killing bosses).
Don't forget that, just like hardcore guilds often have several casual players, casual guilds also often include several hardcore players. Don't forget either that it's not because you're not progressing at a high level that you don't enjoy progression - many guilds that were in Karazhan at the end of TBC still had fun killing bosses here and didn't ask for a free pass to BT, simply aiming at whatever target was in their reach.
But it sure feels much more righteous to show the evil 1 % trying to force their will on the helpless 99 %, than to show a very nuanced spectrum of people with lots of differing opinions.