To add to it: the Terran Hegemony had allowed the rest of the Inner Sphere to start producing some of their own technology by the time the Star League came together, but it was always the industrial and technological heart of the Inner Sphere. Hell, the Star League started as a plot to secure the "landlocked" Hegemony resources their own worlds were starting to run out of after centuries of intense industrial output.
The other future Successor States had their own industrial bases (and the Lyran Commonwealth was already an economic powerhouse by then), but most innovation came out of the Terran Hegemony. And, IIRC, through the Star League the Hegemony also supplied the other states with high-tech
components and tooling which were then used in the manufacture of a million different things across the Inner Sphere. Without the Hegemony, those supplies dried up. And with Amaris' fall resulting in close to a hundred worlds being put through scorched earth tactics, all that industry was mostly gone by the time the Successor States rushed in to carve out the Hegemony's corpse in the wake of Kerensky's offensive. Even ComStar, with better access to Star League and Hegemony records than most, was left with a pretty fractured database to work with.
That's exactly what I was talking about, yeah. It takes a lot of different materials and factories to build just one BattleMech. For a slight taste of how difficult actually
building a combat vehicle is,
here's how much we managed to fuck up in WWII. And that was a World War II tank. An enormously primitive machine compared to a 'Mech. And it still needed parts from over a dozen different companies to assemble. You'd need parts from all across your
Successor State and beyond just to be able to put a bog-standard Locust together, and all that requires not only companies to build that shit, but also DropShips to carry it up to space, and JumpShips to bring it to your factory. Plus more of that to take these Locusts out to their "local" supply depot.
I don't remember any actual hard numbers or even an estimation. I do recall at least a billion people dying in the Federated Suns alone during the
First Succession War, and that might be only referring to people in planets where fighting actually happened. Probably doesn't even mention worlds far from the frontlines where disruptions in logistics and trade resulted in famines or unrest. The Third Succession War was probably the least lethal, since by then the whole thing had devolved into border skirmishes by very depleted armies.