Observability has changed in most other ways we have regressed.

That's true. And while I disagree with the parent comment, ICBM interception remains enormously problematic and likely will remain so until directed energy weapons get really cheap.

Fundamentally the rocket equation and orbital dynamics really fight you on this.

It's a lot less "can't be done" versus "would be financially untenable to build and maintain even when the objective is nuclear defense".

Have they, really?

* Small rockets can now land themselves.

Anything else?