>
the stagnation in some measure spells its doomI'm not sure where you learned about "Eternal September", but it's the exact opposite of what you're describing: not stagnation of culture, but degradation to a pre-existing, high-quality culture due to the rapid influx of new individuals who don't give a shit about maintaining it.
Amazon is going down with Americans precisely because the (predominantly) Chinese sellers on there now couldn't care less about the "nice online garden" Americans created and are instead using up the pre-existing cultural capital Americans had built up for their own selfish advancement—destroying the garden in the process.
But yeah, sure, maybe after the garden has been burned to the ground, something new will replace it. Sounds great.