I do think that accessability is a very important concern for any project and I think that ideally it would be ready prior to Gberg launch.

Having said that I think the primary reason the team is focused on an expedited launch is because until it becomes official Gberg will always be a 'feature plugin' and the majority of the eco system will not prioritizes it.

For example a handful of the plugins we commonly use on our client sites are not compatible with Gutenberg. We have lodged support requests with the plugin developers and the response is more or less: "We plan to look at this once Gutenberg becomes official"