The goal of bitcoin was more to produce a digital ecash prototype that works fully decentralised...

It's not the socialist economy product you're imagining. You can own whatever you want, you could've bought at $220 if you were old enough in 2015 for example - what was stopping you?

If we assume 4 billion people have a constant supply of food and water, then you need 0.00525BTC to have something realistic to hold or use in the future.

For 8 billion people, 0.003btc will be out of reach to quite a lot of them so you could also go with that.

21000000 people can only own 1 bitcoin (population that size is tiny, like 4 new Zealand, less than half of England, 2+ individual Scandinavian countries).

210000000 people can only own 0.1, that's less than everyone in the US.
2100000000 people can only own 0.01, that's less than China and indias cumulative  population...