The long story format has been around for a good minute -- it's not gonna die out anytime soon.
Look at a lot of the mangas you've might've read that have lasted 10, 20, even 30 years. They're still popular, still have their fandoms, and they're passed down generations.
Even outside of that, you have western styles comics that have lasted decades. If not the superheros genres, then it's other comics like the newspaper comics (like Peanuts, Boondocks, Otis, etc.)
One reason now readers more or less become less frequent is that sometimes, those stories can drag on with the same plots over and over -- the key is just keeping the story fresh and exploring new things.
Another reason is that some just wait until the story's finished and then they read it all. Either that, or people just get busy and can't focus too much time on a long series. It happens -- they come and go and then they'll come back to it.
So I wouldn't say the long-story format is doomed. People still adore it and will still keep coming back to it. In any case, if your story's a long format, and you WANT to tell it, why should this stop you?
There's always gonna be possible reasons to not do our stories. If it's not this format, it's the genre, or the topics, or the artstyle. It's just up to US to choose to keep creating despite those things.