A serious question:
Yes, we enjoy and feel a lot reading this. But do you think the manga medium (especially the cutesy chibi drawing style) was what have sent the story and its message across?
Because i have a feeling most of us will get bored out or feel disinterested if this same story was told through...say... an essay format. Or interview format. Most probably won't even read at all.
Can manga as a future medium to send one's own story across? Or as soon as it picks up some popularity (like ...now..) it will immediately saturate the market (and we end up rolling our eyes over "Oh God, another author publish their depressing life story again")
Just some thoughts.
No offense, but it didn't feel to me like a serious question. Differences in opinion, probs. Well... Anyway...
Nagata-sensei did prove her medium to be the the best out of countless testimonies so far. This is, however, leaning to the biased factor of its market value... probably (I don't live in Japan). It's novelty, so to speak, made itself known. (As in an essay type of manga, that is.) Not to mention that it was first published in the web, thus easy access to the notice of netizens. The ever curious netizens. The ever... netizens.
Anyways, there have been published non-illustrated essays out there that are just as engaging, if not more. My say is that people are more attracted to illustrated works for their more direct approach at conveying their message.
Now for your fear of this being a trend, I doubt it will be as viral as you imagine. First of all, it won't be easy to create a satisfying manga about depression. There might already be tons of similar mangas out there but never made it out as big as this one because:
1) They did not resonate with the people of its demographics
2) The lack of real-life feel
3) ...twitter and pixiv, maybe?
Now if many depressed individuals with talents such as Nagata-sensei courageously manage to break these, then I'd be glad if they found some help from within themselves by the end of it. (And then I'll be searching for good ones).
But, the again, I doubt it will be a trend. Maybe for a short while, when the evoked similar thoughts break their shells. But not something anyone with a brush can make, this type of feels.
If I'm proven utterly wrong some months from now, mm... Oh well! xD
Don't worry, though, it won't end the world that way. Trends pass.