They have introduced and removed many things because they have a culture of continuous experimentation, most of it very small. Twitter is much less good at that, and this is definitely not the sort of small, quiet experiment that has done so much good for Facebook. So instead of this being a step toward Facebook-style experimentation, I'd say it's a step away.
I agree this will change how people use Twitter, so I'm not sure why you're saying that in a way that seems contradictory.
Many of your comments have been about how 280 characters won't work well with the way English speakers currently use Twitter. As this won't be the way English speakers use Twitter going forward, none of those comments matter.
If you're just saying, almost tautologically, that users will use Twitter before and after the change, and that use may evolve, sure I don't dispute that.
However, what you started out saying was that Twitter was even more popular in Japan, a place where the word count is higher. Given the context, it seemed like you were implying that this could or should make Twitter even more popular.
I'm saying that's poor reasoning, because Twitter's use in Japan is distinct not for reasons of word count, but because the culture of use is deeply different. There are deep sociological differences, likely driven by the substantially different Japanese culture, plus a different early history of use than in the Anglophone Twitter sphere.
280 may be better for English-speaking Twitter and it may be worse. But Japan proves nothing about English-speaking Twitter because actual use is too distinct.
I'm not saying that Twitter will necessarily be more popular in the US with a higher word count, just that Japan is an example proving that Twitter can have a purpose with a higher word count.
I agree this will change how people use Twitter, so I'm not sure why you're saying that in a way that seems contradictory.