Something to ponder:

Gutenberg in its current form (and future I assume) will encourage content makers to structure their content. Currently the average user just plops content into TinyMCE and hits publish, accessibility and even basic design concepts are not even closely considered.

I have seen users who have had someone design their site with a variety of site builders not able to change a simple piece of text because it is buried somewhere deep in a structure or in a configuration block in a page they have never ever edited or learned how to edit. They are truly lost. How's that for accessibility?

a11y is super important, but I think it will be greatly aided by Gutenberg in the very near future, rather than hindered. There are 2 things at play here:

1) Using the editor and 2) the content it generates.

My hope is that the content Gutenberg will generate will be more accessible than ever before due to the fact that accessible elements will be attached to each Gutenberg block as a first class element. Structured content as a result will be also more SEO friendly as Google adopts the new structured pages Gutenberg generates. This is my hope and one that I hope others share.

As for the actual content creation, this is definitely a challenge, no doubt. It appears that in the rush for a superior experience with new technologies (ie React), the WordPress community and Automattic will be forced to innovate and make the underpinning tech behind Gutenberg accessible. No easy feat, but I am sure it is doable. And admirable!

But remember, this is innovation. This isn't just the latest x.x.x release, but a major release. Innovation does require pushing forward, pissing people off, but eventually advancing and making it even easier to create beautiful AND accessible content. My bet is on Gutenberg creating a more accessible web, not one that is less accessible. We just have to be a bit more patient, constructive and probably contribute in ways we have not before.