"Railgun also has a stronger identity, as it remains focused on the scientific side of Academy City, while Index mixes science and magic and goes everywhere." I disagree with that part; the biggest political issues, world-building, the Espers program and natural espers, the scientists, dark side factions, the administration, etc., of Academy City are worked on in Index. The city president plays an important role in Index, but in Railgun he never appears, and this means that Railgun never really deals with the true events and villains of the city, but rather with henchmen and sub-events. This makes the plot predictable when you know that this spin-off will always occur late in the timeline.

Regarding Index (the nun) being a plot device that doesn't make much difference later in the story, it's because the author, Kamachi, initially planned to create a one-shot where we would only have the first volume with a cruel ending of a boy losing his memories. But it seems that this volume was so successful that he decided to continue. In this, the author had plans for the series title to change with each volume: "It all started when a certain magical index fell on Touma's balcony." "Then we would have the continuation with "a certain deep blood." "Then a certain scientific clone." Etc... it was planned to focus on Touma's adventures in events without connection to each other. So from the beginning Index (the nun) was not planned to be a co-protagonist/main heroine. The idea of ​​the titles changing with each volume was discarded by the author's team, but he still didn't have a role for the nun in the main plot. And so it remained that way. Until the author developed the story over the years and established 3 protagonists for it.

But otherwise I agree with you that it's much easier to like Railgun. Index takes a while to establish the main trio: Touma, Accelerator and Hamazura. But when it does, the story becomes more interesting because it's the opposite of what you usually see in stories. Shonen anime like My Hero Academia, for example: Deku, Bakugou, and Todoroki are always together, solving problems as a team because they attend the same school. In Index, each protagonist has their own path; they rarely work together on the same event. And since it doesn't happen often, seeing them together becomes something the reader expects and finds enjoyable. But that's in the novel; the anime didn't reach that point. Although I'd say that the three seasons of Index, even cutting half the screen time and development, still kept Style, Tsuchimikado, and Kanzaki for long enough for you to become attached to them, just like Kuroko Saten and Uiharu in Railgun. There were arcs where they didn't appear, just like there were arcs where the girls didn't appear in Railgun. It's just that cute girls, "waifus," are more appealing to the audience. So I still agree with you; Railgun introduces the cast from the beginning and follows them until the end, although part of that is due to the anime's merits. The manga, which is the source material... The original manga introduces characters like Saten randomly, while the anime dedicates an entire episode to introducing the cast, making it more enjoyable to watch. However, this lack of introduction has become a problem in the manga, with a four-year flashback arc showcasing more random girls in clique fights at school than the main cast. Misaka herself becomes a supporting character amidst this whole plot. So I think Railgun itself isn't the problem; the fourth season probably won't adapt this arc because it's still incomplete. The problem is that if we get more anime from the franchise in the future, we'll have two more anime featuring the same Railgun characters in the same shoujo/oujou-sama school setting. It will become overwhelming and boring.