Keep in mind that "extinct" in a rules sense doesn't mean completely gone. It might not even necessarily mean no longer produced. It simply means that no known manufacturer of significance is still building them.
In this context, it arguably does make sense, because even in the succession wars era, there are better alternatives, and there's no reason for the houses to make them when they can still make autocannons and lasers. Once you're building rifles, you also need to build ammunition for them, taking up time, industrial capacity, and logistical load. It's a considerable investment of scarce major industries for little tangible gain.
That said, rifles are so simple that if someone decided they had a reason to build them (I can think of a couple places where a rifle might be pretty handy), they probably could retool existing industries to do it with little effort.
And funny note about rocket launchers. The marian version is unavailable until the 3060s, but the primitive version is very available in all eras. I suspect that in this case it's less that people are actually manufacturing them and more that they're so simple that if there's a demand they can be turned out quickly with little to no industrial capability.