Ideally, socialism isn’t just “the government provides for your needs”, it goes the other way around too. The point is to come together, pool resources and combine our strenghts. There’s no free handouts, you give and you receive.
You shouldn’t have to enforce a birth rate cap if the population understand that they need to match society’s capacity to expand and build the infrastructure. You’d announce the recommended number and danger number, and people would organically organize to on average make it, knowing their large family could lead to famine.
The main problem here is
If all basic needs were met (food, shelter, and medical)
That part does a lot of heavylifting there. People only play nice all together when society is working for them, people need to respect the society they live in. When scarcity happens, people become selfish, it’s survival of the strongest, and everything falls off the rails and naturally goes to capitalism and hoarding resources. The population cannot lose faith in the system.
Scarcity and being poor are two different things: when you’re poor, the grocery store is fully stocked, you just can’t afford it. When resources are scarce, the store is empty or almost empty. We’ve seen it artificially with COVID when people panicked about toilet paper. People bought all the toilet paper, some even to resell at higher prices. People do crazy things when you don’t know when you’ll get more, especially food and essential supplies.
Being kind when you’re poor is advantageous because other poor people will help you too when you need something. It’s community, and you basically end up somewhat pooling and managing resources together. But it kind of only works at small scale, because humans build relationships. At large scale it breaks into groups, and groups form even bigger groups, and you have factions and big fights.
For capitalism, maybe it’s not the best fitting word. I mean specifically people will barter and trade, someone will find a way to generate profits, hiring other people for profit and eventually slowly start hoarding all the resources just like regular corporate modern day. Even the concept of taxes is pretty old: you give me some extra grain and I’ll protect your farm. Someone will find a way to position themselves as important and justify taking more than others. Get favors, bribes, make more kids to help maintain the family’s power.