I would agree with this:

“The biggest issue right now is that the REST API isn’t included in core,” a representative from Ashworth Creative commented.

An associate of mine, the owner of a reasonably large WordPress related software house, has a big piece of software already written and waiting to go. It will not be released until it is in core. Having all these beta plugins is of no use commercially, it speaks of adoption being far too risky.

The BIG issue is security. If it goes into core, there is going to have to be a lot of hardness testing, because even now the teenage trolls of the world are already writing tools to hack it. My guess is that Matt M will see this as the biggest possible risk to the whole WordPress project. If this goes in core and goes south ... well, the risk is far too great.