Slashdot lasted maybe 10 years (though still limps on). Digg lasted about 4 years before it started shedding users at alarming rates (and 6 years before it killed itself).
But after 20 years, Reddit still is still gaining users; It's not dying yet.
Reddit has changed so much over time. Reddit of 2006 was very different to reddit of 2008, and reddit of 2013 was very different again. By 2019, it's more or less managed to reinvent itself as an App, trading blows with Facebook, Instagram and Tiktok, almost unrecognisable.
I'm sure many people will put peak-reddit around 2019, but for me, Reddit of roughly 2011 was my favourite, and it's only been down hill from there.
I don't think Reddit can re-invent itself again, only continue to get worse. But I suspect it will still be around in 10 years.