I think there's two ways to look at it:
Is there a reason for another story? In Maximum Ride, the original books were about preventing the world from ending and they gotta stop the evil company that experimented on them and made them. They did that, hooray! But wait there's more books, why is there talking sea serpents? Why is this suddenly about the environment? The stakes aren't the same as before (or greater), the problems are minor. People would definitely buy a book about Harry Potter and the Long Wait at the DMV because they love HP, but it's not going to be as enjoyable as the others. Stories need set-ups so that there's resolutions. In Pern, the thread shows up at regular intervals. We start with learning about the thread and fighting during the current pass. We learn why all those riders disappeared so long ago, then about how those outside-of-time riders are dealing with their new present. Eventually we solve the menace of thread once and for all, hooray! But now we gotta now about the original colonists, right? Each book presented something new that made you wonder more about the world, so the next book could answer that question in a satisfying way. {Full disclosure I couldn't stand the new books Todd McCaffery put out, a big part of this why is that the stakes kinda don't matter since we know it's all going to work out}
Secondly, is the sequel(s) thematically appropriate? The prequel trilogy of Star Wars made sense, since there was great mystery on how the Empire started up, what happened to all the Jedi, what caused Anakin to fall and become Vader. If you look at all of Star Wars about being about the Skywalker family, but also of found family and a sense of wonder and adventure, then the prequels fit the theme. The Force Awakens did, too, but then they totally mucked it up and it became clear it was just for money. The Testaments, the sequel to A Handmaid's Tale, clearly exists because there is fan demand for it, but in-universe it makes sense as to why it exists (because....of fan demand for it lol). If we look at these as in-universe historical texts, being read by academics studying the long-gone country of Gilead, then a story about multiple women and their unique relationships to Gilead would definitely exist, there's no doubt about it. It goes over the same themes in a different way. The Giver, which didn't NEED a sequel in any way, had one with Gathering Blue, and it was the same themes about society and withholding information about The Before Times. They're only barely connected, but it looks at the same themes from a different lens.
I think it's easier to set up the series as "we got a big problem to deal with" or "I have a big goal" (gotta stop Sauron/Voldemort/Thread/Galbatorix/the Reapers/etc or I want to be ninja president/king of the pirates/a pokemon master) and each book/arc is a clear major step that builds up to that than "oh, well I guess I gotta write another one." But if you can pull it off in an intelligent way instead of "well I guess we're going to go to Atlantis now"...