For me personally, I've never minded giving more people access to content by lowering the paths to entry as long as they maintain the challenge there for those who choose to pursue it on a large-scale level. Having run multiple guilds since the start of the game I'm absolutely sure that it's harder to manage a guild running 25-man content than 10-man content so I see a need for greater rewards, but at the same time I think more people getting to see content is a good thing as there are plenty of occasions where good players who have a lot of skill don't have enough time to commit to a 25-man schedule. It's nice for them to see some content as well.
There are just some lore things that you have to sacrifice when translating to a game. I'm sure Illidan probably wouldn't have been killed by 10 or 25 people, but at the same time I would have to guess that he probably didn't die once a week and get resurrected every Tuesday morning by a server reset.
Some guilds might disband and go the 10-man route. Others will want to tackle the challenge of 25-man. Either way there are a lot of "Lore Perspective" things that you just can't do from a standpoint of a successful game so I'm personally fine with the path Blizzard has chosen.