Seems like there's a reasonable parallel here with video content. Before native video, video was always in a flash player, and that was good enough for a lot of people— it was proprietary of course, and it it required a plugin. But it worked.

Now with native video, there are still precious few instances where it's just the browser's native video widget. In basically all serious commercial scenarios, there's at least custom player chrome, navigation buttons, overlays, JavaScript to pause the video and play an unskippable interstitial, etc etc.

I would argue that the current state of affairs for 3D viewers is closer to the current state of affairs for video than it is to the Flash player story of 15 years ago. No, there is no explicit "model viewer" widget, but you could just as easily argue that WebGL canvas is to 3D content what <video> is to h264 and webm content.