I'd say, "Let's look at how this was done," in terms of the evidence you mentioned for your worry.
1. Tales of MU: Alexandra Erin started multiple serials, (something like four or five in total) after Tales of MU grew. At that point, she found it impossible to keep up with her self-imposed insane schedule. Added bonus: she has some kind of disease that actually saps her energy. At some point not only did her extra serials start to post irregularly, but so did Tales of MU--to the point that the audience shrank considerably.
2. Meilin Miranda took a break from her established serial to revise it. That took a very long time. It took so long that she started Scryer's Gulch after a gap of some months after Intimate Kingdom's break--a long enough gap that the regular readers had stopped checking in as much.
In my view, the problems with the "second serial dropoff" has more to do with the schedule break that lost both writers a lot of readers.
In addition to that, both serials drew from complimentary circles of readers (fantasy readers who enjoyed sexuality and not just heterosexual sex). Both serials had issues during the same period, much like they had their growth in the same period with the result that they weren't effectively crosspromoting each other (which they did mostly unintentionally).
Now Worm is another thing altogether.
Worm was a superhero serial that hit when people were discovering an appetite for more superhero stories, but it had its own distinctive spin. Plus, it ran long enough that people who would enjoy it had a chance to discover it while it was running. Additionally, I think it may have tapped into an interest in darker superhero stories that definitly existed, but hadn't been previously obvious.
He did an excellent job filling that niche, but he went on to do something that wasn't related to superheroes. Wildbow did urban fantasy next, and now he's doing Steampunk (well, more like Biopunk if that's a thing). These aren't the same genre that he became known for.
He probably would have kept more readers if he'd done "Worm II: Even Wormier"---except he probably needed a break from that universe and that mindset. I know I would after that many words. He's made the right decision, I think in that he's writing the thing that sounds interesting to him. Bearing in mind that he has an audience, he's in an excellent position to grow again--especially if he writes something long enough for the audience to find him, and if he gets lucky again like he did with superheroes where the topic itself has momentum.
To echo Alexander, (whose comment I apparently just restated, but with more words), the thing that's really going to help Wildbow is to put Worm, Pact and Twig into ebooks where they'll find a whole new audience (and one that pays better).
EDIT: You know what? I'm going to make this already long comment even longer by adding my own plans for what I'm doing next.
1. I'm going to revise Legion of Nothing section by section and turn it into ebooks while it's still going, hopefully drawing ebook readers to the ongoing serial (which worked well for Drew Hayes).
2. If I do a second work that's totally unrelated, I'm going to finish the first draft before posting it on the web in sections--thereby stopping it from interfering with Legion's consistency. And then it will get revised and turned into an ebook.
Legion's function was always intended to be something that's on-going and a kind of calling card that people can find and then become aware of other things I'm doing. It's got a planned out ending, but it's not going to appear for a while. In the meantime, I'll be completely happy if it turns out to be my best known work for a while. Sadly, the other stuff I'm working on hasn't made it to the web yet, but I'm confident it will.