Australia tried to follow this with gov.au, and it was a bit of a debacle. It ultimately didn't work because it the agency driving it lacked a stick and couldn't coerce agencies to give up their branding/content. Agencies couldn't see the financial incentive -- they'd have to give up their funding and control, and that just wasn't appealing at all.

Frustratingly the idea will be dead for a good time to come now. For an agency to do this, they need a stick -- I believe the GDS had IT spending levers they could pull to control how money was spent across the entire public sector. This made them quite powerful and ultimately allowed them to say "if you want to publish your agency's content, it has to be on our platform".

The end result is a far better UX.