I'm not sure about rankings for WSM, but Fuuka really isn't any less popular than Kimimachi was book sales-wise. It's been doing the usual 70-80k per volume before disappearing from the charts, which is about the same Kimimachi did throughout most of its run, even after it got the anime and what not. Basically, for someone like Seo it just seems like he's got his faithful audience and things don't change too much in terms of sales.
Also, I really don't see we taking another 30 chapters for Yuu to get back to being how he was. Or, rather, to at least start walking a new path after accepting what's happened. That seems like way too much time to spend on something like this. If I had to say, the fact Suzuka and Yamato weren't shown at the funeral is because they'll be playing a part in getting Yuu back up once again, probably because Fuuka is bound to have commented about him to her family before. She's the talkative type, so I'm sure the "very kind friend" and/or boyfriend was mentioned to both of them -- and she'd been her happiest as of late, which I'm guessing is something both Suzuka and Yamato will also acknowledge as it being due to Yuu's existence and support.
Anyhow, I think it's a pretty interesting setup he's got here. It's very reminiscent of Mitsuru Adachi's stories, where the death of a loved one is usually the catalyst that brings up changes to our main character(s) and pushes them on towards a certain dream/objective, and that kind of story, when told right, makes up for some of the most earnest drama I can think of. Of course, Adachi is a master at his craft, and I've no idea how Seo's gonna pull this off, but at the very least whenever Kazama was brought up in Kimimachi it made up for some good pages. The visits to the grave, the finale, they all came across as very emotional scenes, so I'm willing to trust him a bit on this one, too.
However, I can obviously understand why some people would be less than pleased with Fuuka having to die for this. I mean, depending on how big a role Yamato and Suzuka play out and how relevant a character they become for this story, people will pobably calm down, but it's still a weird decision to kill the daughter of one of your most beloved characters in order to advance a new story. It risks, just as it did, pissing off a part of the fanbase, and arguably you could have done it with a "new" girl just about the same thing. On the other hand, though, Seo has been playing around with his "Seoverse", where you can catch glimpses of other characters' futures even after their stories are over, and as such I guess an unfortunate accident like this is not impossible -- it's something that could happen to anyone in real life. Looking at it from that prism, there's a lot you could do with Yamato and Suzuka after this, even further enhancing their bonds after having to comfort each other in such a desperate period of their lives. And for this, at the very least, the fact that the readers are familiar with the past and struggles of these two would obviously be required for Seo to make the most out of it.
All in all, though, I'm left very intrigued, and definitely interested in how he's going to play his cards now. I guess it helps that, unlike most people, while I did enjoy Suzuka, I was never too big a fan of her character, so I'm probably taking this best than most people. I imagine that if something like this were to happen to Daiki, Yuzuki and Haruto's son, my reaction would be far less understanding, as I have a big fondness of that couple and their story.