I dare say either of those are a tad more complex than a PPS-43 or a sling-shot powered jam tin grenade.
True but they are something a Tech B planet could build. A Tech A planet would be making Gatling Guns. Presuming of course they're more technologically advanced than sling-shots.
As a point I made earlier, you still need the blueprints and chemical formulae in order to properly reproduce them. Even reverse engineering takes R&D, and those institutions were smashed such that we didn't see anything new for over 150 years. Everybody was so focused on what they could maintain, that even researching backwards technology wasn't feasible.
Yet there is a rather large difference between the most primitive of cannon and the cannons used on 21st-century tanks. Especially when we have people who have time and resources to play around with doing recreations, i.e. R&D. Both metallurgy and chemical formulae for both propellant and explosive creation are still needed, and that means R&D if you don't have them on hand.
There's a huge difference between, "I have a permanent stick to bonk with," and "let's recreate something that requires an industry to produce."
That presumes that they don't already have those things. Rifles were made and used for centuries. Sure there'd be some R&D but they're not starting from scratch. They've been in production and use for centuries so they would be known.
Rifles have an availability of F during the Star League Era. They don't go extinct until after the 1st Succession War around 2825. So for most, they'd be hard to get but they could be obtained. Their going extinct after the 1st Succession War doesn't make sense. The planets making them wouldn't have been low military targets. With all the disruptions of the 1st Succession War making it difficult to get Age of War tech I would think they'd start making more Rifles, not less. Even if by some chance that all the manufacturers were lost Rifles date back to pre-spaceflight. They're low tech enough that they shouldn't need a NAIS to research and build them. Sure there would be some R&D but they could be built. Otherwise it'd be like saying we can't build flintlocks now which we can.
As for other replacement tech, it isn't until after the 2nd Succession War that everyone works on just maintaining what they have. I would think that they would have been trying to come up with replacements for tech lost before them. I lean towards blaming Comstar for why they didn't manage too. Still, I wonder what was used before Beagles and Guardians were introduced. I also wonder how EW Equipment is extinct in 3046 but has a Availability Rating of E during the Clan Invasion.
Because the universe has zero reason to have them retconed into that era. The time period is fine they way it exist now. Zero good reason exist to change.
Except a lot of tech has already been retconned into existing during that time period. We haven't seen them but they exist. The question was why some things aren't available.
Rockets are so useful they should be more available than they are.
Rifles are low tech and cheap enough that they should be available, if rare.
Chemical lasers are useful enough, and have existed since the 1990s that they should still be available if rare for the same reasons Clan Hell’s Horses brought them back.
Primitive Prototype Weapons, could possibly be found in some lost cache or in the deep periphery but I can't see them going back into production in the Periphery or Inner Sphere. If the factory is capable of building autocannons they'd go with standard versions.
Rail Guns. I can see some trying to bring back rail gun as replacements for gauss rifles and Comstar stopping them.
Ancient Missiles, they're too big and heavy that I can't see anyone producing them over standard SRM/LRMs.
Would that effect the Succession Wars? Sure, but I don't think they'd change things that much. We've had a lot of tech that existed during the Succession Wars for years and it hasn't really changed the Succession Wars. Even if we were to get a TRO filled with that tech it'd just give players some more options for their games. Adding a few more weapons isn't going to change that.