I wonder for such abandoned projects with non existent supply chain/know-how/documentation, whether there is any benefit of trying to revive them instead of starting from scratch.

I would bet good money it's one of two things:

a) Some clearly preposterous order that originated with the demented commander in chief, and they're just going through the motions. Same concept as ordering steam catapults put back on aircraft carriers.

or

b) Intentional misdirect/bait to send various groups of people off in a wrong direction as cover for something else entirely.

Up until relatively recently, the Saturn V (and the F1 engine) was the most powerful rocket ever made, even though it's almost 60 years old. Now I believe both SLS and Starship are more powerful.

So this led to the obvious question: why didn't we just recreate the F1 engine and the Saturn V? It turns out there were lots of reasons:

1. Materials science has changed. You can't necessarily make the same materials you made back then. And substituting them might have consequences on certification and testing that would make it much more expensive than a simple recreation;

2. We didn't necessarily know how to. Surprising as it sounds, the Saturn V program was pretty rushed. The engines and rockets were essentially handcrafted. Lots of things were documented but not necessarily everything. Even if it was, has that documentation been lost?

3. Loss of expertise. There's probably hardly anybody left who made those engines. Any complicated production line relies on reinforcing and building expertise through training and repetition. You'd have to build that from scratch. This is a common problem with any major weapons program. As soon as you stop making a particular weapon, it's almost impossible to restart it very quickly;

4. We don't necessarily have all the machining and secondary processes that existed then;

5. Advances in technology mean we can do better things now. For example, we can 3D print parts that we had no way of making them previously. It simply wasn't possible. This has come up in commercial aviation where modern aircraft (eg 787/A350) and their respective engines make heavy use of 3D printed parts, something the 747 could never have possibly have done;

6. Even if you can make all a complete system, what's your production capacity? The capacity for Saturn V rockets was quite low and because it was essentially bespoke, it couldn't be scaled. One of the impressive things about the Falcon 9 system is just how many of those rockets SpaceX could produce, even before extensive reuse of first-stage boosters. Capacity is something that almost has to be built in from scratch and it impacts everything. For example, a bunch of weapon systems have been depleted by the Iran War (eg Patriot interceptors, Tomahawks). You can't just turn a knob to ramp up production meaningfully. And where you can, it's incredibly expensive.

This is also a big part of China's current manufacturing advantage. There's an awful lot of standardization and regional specialization. You need a part? The factory that makes it is probably down the road. So certain things get made in Shenzhen because everything you need is also made in Shenzhen. This also means you can react very quickly, something a factory in the US could never do because it lacks that ecosystem.

They discovered all sorts of weird things when they made the original Blackbird. For example, titanium is so hard that they had to use special tools to machine parts like the panels. Well, in doing so they discovered that using cadmium in some tools to make them operate at higher temperatures and against harder materials would "poison" the titanium fatally.

So if you built a new hypersonic airframe now, would you use titanium? Do we have better alloys? We've had a lot of advances in single-crystal manufacturing for metals that have, for example, made jet engines incredibly reliable. And this btw is something China has had massive difficulties reproducing, at least for now. And they've spent decades trying.

As a simple example of this, there was a guy who went out of his way to make an electric toaster from scratch [1]. This is a relatively simple device but it shows how many inputs there are, which each have production lines (eg steel, aluminum and/or plastic). Now imagine what goes into a military plane that needs to fly at Mach 3-4.

[1]: https://www.ted.com/talks/thomas_thwaites_how_i_built_a_toas...