Only just how much more complicated the original SDS system was.
So it is more than sticking HNPPCs in old missile silos? Or is it just the space based parts of the SDS that are that impressive?
Based on what JHS:Terra says, the ability to build Castle Brians is gone. Kaput. And frankly, I agree. CB's are incredibly intricate, with each example almost one of a kind. Plus, building them when slim resources can be directed elsewhere seems like a bit of a waste.
Now, that being said the original SDS system was incredibly complicated. It was a totally automated system from the M-5 Drones down, all the way to the robotic ports that serviced the vessels. However, it was the control system of the SDS that's irreplaceable. The artificial intelligence of the SDS really was cutting edge. Perhaps, the most advanced piece of equipment ever designed by the Terran Hegemony and one of a kind. Unique as the dead scientists who designed it.
The Word's "low-tech" successor was based around small, human controlled drone systems that were either directed through a local control ship or station. However, the human element was present in heavy doses. People were necessary for everything, but it allowed the "punch" of the SDS (Drone DropShips & Fighters) to be unmanned. Given a little more time, the Word would probably have been able to blanket Terra with an unbreakable system by late 31st century standards.
For the Republic, they have an abundance of WarShips, (more so than the Word at any rate) so they'll have plenty of mobile control and/or carrier vessels (although using manned warships as carriers should be a secondary option.) The Republic should also have plenty of manufacturing capability required to mass produce drone fighters. They own more factory worlds (even damaged) than just Terra. Forget about the drone Dropships or WarShips, which take too long and cost too much. Drone fighters can be built with ground facilities, so you'll be saving yard space for other projects. You don't even need to build space-based facilities or create the Star League's integrated infrastructure. And forget about building hardened static SDS facilities planet-side. Those are luxury facilities. Meaning, you build those when you have the luxury to spend time, money and resources on them. I'm not saying your existing static SDS facilities should be ignored, I'm just saying that for every hardened HNPPC silo built you could have just as easily built a dozen or more drone fighters....each one capable of maneuvering and then attacking or ramming something as valuable as a 'Mech carrier or WarShip.
I'd just build these little ships en-masse on each planet with a couple of drone controllers. Any invasion force closing with the planet will have to face the swarm. This could provide the Republic with an overwhelming aerospace advantage. Coupled with the Republic's mobile reserve, the space assets of the Great Houses will face being pressed against a planet's SDS and Republican WarShips at their rear.
As for manpower. I was referring to the Republican military. In its infancy, the RA is very much a one trick pony. For a couple of years, it can't afford to sustain heavy losses. Many of its soldiers are hardened veterans, but until the university system and reconstruction are complete, any major incursion into the Republic has to be handled carefully. If bled too much, the Republic's already weakened position weakens even further...making additional invasions a more likely proposition.
Again, don't underestimate the fear of the word "SDS," or the benefits even a partial system could impart to help bolster the Republic military and recovery effort.