The way Riot has been making these stories have been complete one-shots where they won't be continued at all and don't allow for them to continue. Although some books don't have sequels they establish an enormous amount of stuff to tell a complete story or not, doesn't matter if the story is finished because a ton of stuff gets established. In the short stories Riot has been making basically nothing gets established.
All forms of media, story telling, and literature can be compared to one another because they're all literature story medias. One could compare the amount of world building within a book compared to a movie. For example, simple things like showing background characters in the world do something behind a main shot can establish world building. Considering a bunch of other art stuff, all literature things can be compared because at the end they're all telling a story and that story telling can be compared.
I know that we will never reach an end for these characters. It's like that for all games of these type. That doesn't mean the characters aren't allowed to progress and stuff. Hell they don't even NEED to progress. As mentioned before and in OP. Although it is still pretty lame to have stories with little plot progression just GETTING stories and drawing out the character a little bit more would really liven up the world.
Your comparison doesn't work because in this case, lack of future development is a negative trait to have. Sure this trait would be fine if the story was finished, an example could be for The Last Airbender which built a powerful world and ended the story. However in the case of League where the story WILL never end it becomes a horrible trait because now the world just stays static and nothing changes. In a world where nothing changes there is no sense of problem and there is no story. Also I don't think the thought about static lore being traditional is completely nonexistent and not a defining trait. Honestly it's just something that moba people don't think about. Like they have no reason not to continually update the world aside from money or low popularity (a lot of people aren't playing for the story). It's not traditional. It's just a bad habit, a horrible obnoxious habit.
EDIT: ALSO ALSO. We don't even NEED the CHAMPIONS from league of legends to progress the story. Stories like Progress Day and City of Glass and Iron and so damn good and they champions just make a small cameo in them. I talk about this here: https://boards.na.leagueoflegends.com/en/c/story-art/szdlgFjV-rito-more-stories-please
Story-wise I could argue that having different regions and characters coming from different places is traditional to MOBA lores because if everyone came from the same place that'd be pretty weird. I can't say that lack of story development is something traditional because that's just a huge problem with a majority of MOBA games.
I'm on my phone.