Unfortunately, because Blizzard decided to use the same achievements for both 10 and 25-man raiding, it's somewhat hard to disentangle exact statistics. But I believe there are some clear trends in what data we do have.
Take a look at these graphs, from wowtrack.org, which display where guilds are in progression. In particular, mouse-over the 25-man/10-man lines in the legend (upper-left corner) and watch the graph change.
You'll see that there's a clear difference in the shape of the demographics for 10s and 25s. 10s are shifted to the left and have a much larger proportion in the non-heroic area before sloping down. 25s, in contrast, display something close to a binomial distribution with slopes down to zero on both sides of the peak.
What this graph indicates to me is that no new 25-man guilds are forming!
Instead, the 25-man guilds appear to be the same 25-man guilds that existed at the end of the last expansion, and they are moving through the content more or less as a single wave. Note that the median 25-guild has one or two heroic modes already down, making it very likely that they are experienced guilds.
In contrast, 10s have a lot of guilds clustered on the left side of the high point, indicating that there are a lot of newer guilds there. And if you look at the numbers below, you see that there are about ten 10-man guilds for every 25-man guild.
And this makes sense, to a certain degree. Where would new 25-man guilds come from? A guild new to raiding would start raiding with their first 10 raiders. There's no need to move up to 25s. Maybe a large guild with multiple raid teams might merge some, but then you have issues where Team A is several bosses ahead of Team B, or too many raid leaders, or conflicting schedules or loot rules. Why break up three functioning teams to create one team which may or may not be successful?
But if no new 25-man raid teams are created, then the number of existing 25-man teams will slowly fall just through everyday attrition. In the normal scheme of things guilds die, and new guilds are born to replace them. That's what's happened all throughout WoW's past. But now, no new 25s are appearing to refresh the blood.
That's why I think 25-man raiding is doomed. Not because of any inherent negative quality in them, but because I just don't see where the new 25-man guilds are going to come from.
Given the path we're on, I would be surprised if 25-man raiding existed in the next expansion. The number of existing 25-man guilds will probably be less than half of what it is now, just through regular attrition.