I think the only way Reddit would truly become irrelevant is if a website that filtered content to users like Reddit does began to rival it like Lemmy for example. They kind of act like a unified login for multiple unique forum pages and it's a bit easier/simpler to sort than something like an XDA-Developer thread where you can have hundreds of pages of comments asking the same question. I feel like other sites that rely on users to follow other users or tags get too disorganized and everything kind of starts to blend in with each other.

This is obviously my personal opinion. I just don't see myself using something like Twitter or Instagram even when a majority of social media sites are trying to do and be everything at once.