I'm an SEO and content consultant, and clients often fail to send content on time. When this happens, I try to push the project forward by creating the content for them.
Create a content brief for the landing page or article and get the client to sign it off. Like that, the content is well researched, and the client knows what the structure will be like. 
Build a tone of voice and branding document aligned with that specific business - make sure AI/ writers have direction.
Use AI to write the content, but spend quite a bit here to make sure it's optimised, humanised, and brand aligned. Also, ensure this is complemented by highlighting the client's uniqueness and expertise.
Get the client to review it.