No, it's more predictable because
1. BBS is supposed to be about Xehanort
2. Xehanort was the only one in the vid that didn't show up, even though the game is supposed to about HIM
3. Theres a masked figure
Terra's appearance, the fact that his eyes change color, and the fact that blue(Terra)+orange(MX)=brown (which is the color of Xehanort's eyes), the parallel between Riku and XH, Xemnas calling Aqua an "old friend", the possibility of Aqua being the 14th member, among other things, trumps that.
And the first thing most people think is "that's gotta be Xehanort"
Pfff, I've seen more people thinking that it's Terra than DS in my stay here, and I'm pretty sure that you have too. It's just been recently that people have been thinking outside the box and contemplating the DS possibility.
While thinking Terra becoming part of Xehanort takes more thinking.
Because it's so difficult to come to the conclusion that MX (who is so much like XH) posseses Terra, resulting in the mixed appearance, right? That's probably the first thing that comes to a lot of people's minds (some of them change their minds later, but whatever). Whereas for DS you would have to come up with an explanation as to why he is so much like Terra and MX.
I of course have come up with a few, but they required much more time to come up with than the simple, common possession theory, which would have taken me 5 seconds to figure out.
As you can see, most people in these threads don't believe it as much as DS being Xehanort proving that DS = X is more predictable than Terra being X.
1. This thread does not speak for everyone.
2. One of the things that makes them think that is the Terra battle, which is a complicated issue.
3. Right now there's more stuff that can be interpreted towards Terra being Xehanort than DS being Xehanort.
Plus, DS being Xehanort is just way too easy.
DS pulls off his mask, he's Xehanort. Done.
Right, so then why would he look a mix of Terra and MX? You just jumped over that.
Plus T + MX = X makes for better story,
You never shut up, so telling you to hush would be useless.
regardless how cliche it is.
Think about it, it's either mx's apprentice follows in his footsteps, or a good guy, Terra, becomes the one thing he hates the most thus equaling more drama~
Drama is more cliché, and more predictable. Which bites. And DS being Xehanort might not be that dramatic, but I kinda prefer it more, because it means that Terra, Aqua and Ven failed, and MX is still out there in his apprentice, and essentially their sacrifices were for nothing.