From Matt Cromwell in the thread:

'Knowing how negatively the iFrame implementation is with regard to plugin/theme assets, I think this should give the project pause. The real answer here is WordPress shipping the Fields API https://github.com/sc0ttkclark/wordpress-fields-api -- without that, implementing metaboxes effectively with Gutenberg is virtually impossible if we care about performance at all.

If we really aren't shipping Gutenberg until it's "ready" then I say we should be putting equal attention to the Fields API so that metboxes work correctly and seamlessly with React in Gutenberg.'

Replied by Tammie lister with:

'If not many are willing to rewrite their plugin/theme for Gutenberg, I don't think they would be willing to do so for a Fields API.'

I think this is seriously underestimating the support for a Fields API. Especially considering that the biggest obstacle and concern for everyone is Metaboxes.

I'm pretty sure, if given a fields API, most serious developers would be very happy to redevelop their plugins and themes for it. And it would pave the way for Gutenberg to not be such a clusterfrack.

p.s. Craft is looking more interesting by the day.