We’ll have far more infra and a far lower population.

Nope. We will have a lot of older people who need the help of younger people in order to get things like medical aid, food and so on. And we will have lesser and lesser young people to provide these goods. To make matters worse, the younger generation has to deal with issues like maintaining infrastructure, building new technologies, and fighting the unwelcome effects of climate crises.

In a word: If utopia has a chance to happen, it will make us wait for almost 2 decades. No guarantee that utopia will ever happen. History knows not one example of population declaine without a collaps. Captialism isn't the issue here.