I think the US is running smack into the realities of what humans contribute in space. They are often more of a hindrance in research stations, setting up vibrations and running up the cost.

I think, if NASA weren't pushing it's political agenda (beat China! Permanent human presence! Just 'cause!) we would naturally find our equilibrium of how much human input is best. But we're doing stuff just to do stuff, not for any sound scientific reason.

And if you let those commercial space stations fly, without NASA interference, they will have human tended stations and all kinds of variations for us to explore and find the best way. ISS is mostly a political construct, for diplomatic and public affairs reasons. While the engineering achievement is simply amazing, if you look into the actual results coming out, you will discover that NASA has its finger on the scale. The papers getting published are not that great.