This may be my perception talking, but it seems interest in 25 man raids overall has gradually gone down with the introduction of arena gear and how easy it is to acquire badge loot. The draw of 25 mans is primarily to see things like Illidan and Kil'jaeden, and the gear is secondary. To make the same zones able to be accessed in an easier form seems to me that it would be just make the 25 man scene worse.
Will the encounters differ between the 10 man and 25 man versions? Yes, probably. Most likely, even. However, it is still, overall, the same content. You can turn Ragnaros blue and make him cast Gravity Lapse and Polymorph and in the end, you're still in Molten Core. The fight is different, but he's still Ragnaros and you're still in the zone you were the other day or even earlier that day.
The draw of gear has lessened dramatically over the course of The Burning Crusade, because raiding isn't the only source of that gear. The draw of the content will fade, because even if the fights change you're encountering the same guys in a setting that looks identical.
Personally, I think unique content in the 25 man would be the only way to really make this work. Bosses you can't see in the 10 man. Scenes or events you can't see in the 10 man. Things other than gear that you can take out into the world and have an impact. Nothing says Blizzard isn't doing this, it is simply something I feel is necessary. We don't know either way, but I really hope we see something like this.
Now, I won't go as far as to say this is the end of raiding. I don't think it is. If you're going into Wrath with an established raid group that was successful in The Burning Crusade, you'll do just fine. The impact will be on the recruiting end. If you lose an important member of your raid, replacing them will become much more difficult. The draw of 25 mans will be minimal.
Heroic 5 man dungeons still have some draw because they only take 5 people and less than an hour of your time. A heroic raid is going to be a much larger time investment and it's much more challenging to schedule and organize. Better gear and Badges of Justice 2.0 just aren't going to be enough to fuel a raiding community. Unique content is going to be a must, or 25 mans are going to end up in a tight spot, I think.