That’s a great question. From what this article describes, the movement to reclaim space for pedestrians and cyclists started within civil society, not with city planners. Of course at the time there were still people hesitant about these changes, but there was a momentum.

> In the 1960s, Dutch cities were increasingly in thrall to motorists, with the car seen as the transport of the future. It took the intolerable toll of child traffic deaths – and fierce activism – to turn Amsterdam into the cycling nirvana of today

https://www.theguardian.com/cities/2015/may/05/amsterdam-bic...