So is it wrong to assume that Comstar is responsible for 99.999999999% of the Lostech? I mean the chances for the Houses to destroy the facilities making the product, anyone that knows how to take care of the facilities, and the information to fix said facilities seems almost nil. While Comstar capitalizing on parts of those happening seems like 100% chance.
No, ComStar was responsible for some punctual, targeted technology losses, but most of it was just the Inner Sphere being too nuke-happy in the first two Succession Wars. The more advanced the technology, the more of an industrial "tail" it has, so any disruption to the chain means you can't produce the product anymore. And once the surviving factories are shuttered and forgotten, or repurposed into something else, it's easy for data to just rot away. Remember, BattleTech was made in the 80s. Storage media at the time was tapes (it still is in some applications) and it wasn't such a networked world as it is today. It was very possible to important data to be lost and the concept of off-site backups didn't seem to dawn on FASA back then.
Also, it's not like the Inner Sphere completely lost everything. What they had left by 3025 was still ridiculously advanced. Fusion engines were still being made. Standard 'Mech armor is a very complex composite of many materials that requires very specialized procedures to produce. Autocannons were still firing without jamming even if the arm they're mounted on was used to punch another 'Mech. Even the humble Small Laser, the tonnage-filler weapon in the setting, is a free-electron laser the likes of which we can barely imagine due to how sturdy and reliable it is. The IS lost access to its most advanced technologies from the Star League days, but overall it still held on to more tech than the Star League had starting out.
The real problem was disruption of supplies and destruction of facilities. Sure, your Gauss Rifle production line might still be intact after the Second Succession War, but if the factory two systems over that produced the rifles' superconducting coils is now a smoking crater and no one had an updated, reliable copy of the blueprints, you're hosed. And even if you
had the blueprints and the resources to build another factory somewhere else, if the rare-earth mining facility that produced the exotic minerals required to create those coils was gone, or if the branch of the company that built the control circuitry for the Gauss Rifle magnets got atomized by a trigger-happy Kuritan naval officer... guess what, you still can't built these damn Gauss Rifles and your production line might as well be repurposed for something else that you have both the know-how and raw materials to produce.
ETA: as for why the Successor States didn't go digging for the data... well, BattleTech is this weird Capitalist Paradise where damn near nothing is actually built by state-controlled factories. So even if one company had the blueprints to build something a Successor State wanted, and another company had the factory that could put those blueprints into production, the Successor State couldn't usually just
order them to get together and make it work.