The only problem with Cardboard was that Google didn't it follow it up quickly with a real standalone VR headset, nor did anybody else for that matter. The Oculus Go only came out four years after Cardboard. And it wasn't until Quest1, a whole five years later, that we got 6DOF tracking in a standalone headset (Google's own Lenovo Mirage Solo can do it too, but that feature never made it out of Beta).

The thing that shouldn't have existed was Google Daydream. It brought no real advantage over Cardboard, yet was a whole new completely incompatible VR platform from Google, incompatible in both direction no less. Why they couldn't just implement the additional features on top of Cardboard I'll never understand.

Ironically, Cardboard is still alive and you can find some apps that still support it (e.g. Youtube). While Daydream is dead and can't even be used on modern phones.

That aside, content was a huge problem for Cardboard and other 3DOF headsets. VR video still looks great on them, but it took years before there ever were any real consumer VR cameras. And a whole lot of time was wasted with VR360-2D content, which looks utterly mediocre in VR, while good looking VR180-3D got very little attention for some reason.