Personal opinion: shred it.
Your main argument for not shredding it is because it'll cause loss of information for the people looking for help and stuff like this, right? But Reddit will end [as you said] walled in or destroyed or become such a hellscape of noise that it won't matter anyway.
And the more content that you keep there, the more you're encouraging other people to contribute with that shithole. Their content will get lost too, for the same reason as yours; so by removing it now you're preventing the loss of future ancient wisdom.
Also, if shredding it, edit the comments beforehand. Otherwise they'll be reinstated by Pigboy and his Trotters. I personally simply replaced them all with garbage and left the garbage there, but it's up to you.