So the list of weak or unbelievable aspect of the BT Universe begins.
1. the fact that the Innersphere powers can fully retool from 3025 tech to 2750 tech in less then 20 years but can seem to mass produce clantech in over a hundred years.
Actually, there's a fair amount of Clantech production by Late Dark Ages and ilClan. In fact, dare I say the distinction between Inner Sphere and Clan tech is nearly lost in ilClan, considering how many designs are mixed-tech now.
4. that fact that with the size and population of the Innersphere during the Starleague, that all the people that knew how to run it would be killed off without training others.
Not all of them were. Theoretically, the Star League's infrastructure
could have been restored... except that most of the surviving Star League administrators had gone to work for the great houses during and immediately after the Amaris Civil War. Turns out, when your paychecks stop clearing on time because your home office is a crater, one starts interviewing at new places; especially when those places are offering mighty lucrative deals. Once time came to potentially hire them back, they would have been comfortably settled into an office run by a much more stable employer. Said employers were already planning on abandoning the Star League and had begun making moves to sabotage their neighbors and carve out as much territory as possible once the pretext dropped, so they weren't really eager to help put the League Back together.
tl;dr: not everyone who could run the Star League died, but most were either too depressed to try and/or had been hired up by the Successor States for the much more profitable venture of stealing from Capellans.
7. the fact that the Successor States some how forgot how to build warships when they were invented in 2300.
Many tried, and many WarShip programs actually got pretty far. ComStar had to be extra diligent about creating "accidents" within WarShip programs.