How many leaving would prevent the rest from being able to continue to raid 25? They may desire it and love 25man raiding with a passion, but cannot.
When I first discovered the Cata raid-changes post I was delighted at the thought I would never have to deal with 5-10 "extras" ever again. Now that I think about it, I really loved the scale of 25-man raids much more than 10-mans.
I predict that the initial incarnation of Cata raiding will be "the death of 25-mans" that everyone is heralding. I wouldn't be at all suprised if 25-mans make a strong resurgence after the first teir of raids, however. Guilds may look different, and will likely be composed of two or three very strong 10-man guilds combining into one 25-man. 25's won't dissapear completely though, and I would say that the community might even benefit from this mixup.
Some will choose 10s and some will eventually choose 25s, but the best part about these changes is that giving players the choice forces them to realize what it is that they actually do like better, probably increasing their contentment with the raid their in. Maybe?