The arguments for more humans always sound like a Ponzi scheme to me: we need more people so we can support existing people. There’s plenty of downsides of having more people on the planet. And tech seems to be making it so there will be less and less a need for more people anyway, for better or worse

There's potentially an argument for a ponzi scheme for one-ish more generation after which robots can do elder care and it's not necessary anymore.

Japan already bet on it and the robots haven't materialized, so maybe it's a bad strategy or maybe they bet too soon or maybe it will turn out they did it at the right time.

Every modern economy, socialism included, requires a growing population.

You’re right about the main benefit of population decline though. It gives Nature a needed break