I think the reason we see modern fandoms not living long lives (although! although! that's kinda an illusion, because we like to compare fandoms that aren't even 2 years old with fandoms your grandma could have been a part of) is that there's a certain hostility towards people who can't really keep up with new releases.
There's a sort of pressure to keep up with watching the newest shows as the episodes release, but not everyone can do that, sometimes because the episode releases too late/early for people, some people don't have the time during the work week or some folks (like me) will rather wait for the show to wrap up, so we don't get jumped by things we don't like. Other times you have to wait until someone pirates the show to see it. Or you have to wait for a paycheck and/or a sale to play a game. Which can lead to one loosing interest because everyone is on the train and you've already missed it. Or in case of books you may have to wait months for a translation, because either your English isn't that good/you aren't that confident in English or the book is originally written in a language you don't speak.
That's kinda why indie pilots are successful, because it's much easier to find the time and space to watch at most 30 minutes of YouTube animation, that's roughly how long some people's lunch breaks are.
Old shows/games/books tho? Readily available, some of our parents already have them somewhere (and may have been introduced to them through them) or they rerun them on TV, easy to find in most internet archives and piracy sites. Go more often on sale and tend to be very cheap when they do + the fandom is already built around the fact that there are newbies coming in and most spoilers are reduced to references that are then luring the newcomers deeper, because they want to be on the reference or the spoilers are so well known now, that even casual non-fans know them (like, almost everyone knows about "the cake is a lie") and stumbling on a spoiler is less like stumbling on a mine and more like tripping over something. You may fall and hurt yourself or you catch yourself and haha it off.
And hell, sometimes, spoiler is the hook.
Not to mention that a lot of older fandoms keep the fandom etiquette alive and well. Not like the new fandoms don't! But it's a little bit more of an issue, more due to the people, who are the 1st settlers of new fandoms being on the younger side, as teens often have more time on their hands than adults, who tend to be very excited about experiencing fandom and in that excitement forget themselves.
Like, here's the thing about fandom, when you're young, you can get absolutely latched onto a thing and get REALLY alerted to someone else talking about it or even breathing in its direction. And then you see them doing something different with "your toys" than you and so you take it as an offense. And in that state, you may do something irrational, like leave a hate comment.
It's not like I've never been in that space as a teen, however it is still, really bad to leave hateful messages on other people's posts. So like - type the hurtful thing into a notes app. And leave it there. Do not visit the post. Ever, unless you have something constructive to say. If getting a hate comment would make you think of that person as an evil bitch, the person you wanted to slap it with would think the same about YOU.
Also obviously, don't spoil people that do NOT consent to it. I know you really want to talk to people about the new thing, but you have to hold your horses, until they catch up to you.
You can still yap about it into your notes app and then post it once you know everyone has caught up!
