I think it'll be a slow decay. I'm in the same category of long time user that deleted their history and isn't going back. Reddit has lost a lot of accounts like ours.

In the short run Reddit has already lost a lot of its 'cultural history and identity' (sounds dumb I know but I think the terms apply). There were a ton of 'inside jokes' and reddit history references that made it feel like a broad community and there's probably not going to be a critical mass of users that perpetuate that feeling much longer.

In the longer term the loss of personal mod and active user investment in their subs will start to show. Subs will be poorly curated and become dominated by whoever is loudest and angriest. Reddit was fun because it had a huge amount of engagement on any topic, but that was tempered by the ability to find subs with active moderation on topics you cared about. Now they're going to have to deal with all that desire for engagement but with nothing to keep it on the rails, which will mean the engagement will only remain desirable for those that don't want rails. But once they get their way they'll get bored too because they'll only have their own anger to engage with.

Or kbin! Or Fedia! Or any of the platforms. That's the beauty, they're all correct choices and (to an extent) all open to any user.