You're overthinking it I think. It's less about actually stopping someone with the determination and skill than it is just the random idiot that wants to make one.

Laws like this aren't about stopping something entirely. It's about making it more difficult. A "minor inconvenience" to you may literally be a brick wall to other people. Or it may just be enough of an inconvenience to make them realize it's not worth doing.

Your last paragraph is basically explaining what this is for. Its really less about safety or anything and more about ensuring the profits of weapons manufacturers. So, you're right that it's not going to stop a determined person from printing parts. But, that's not who this is for. Even if they market it that way. That's just political marketing.

Some responsible citizen might find out they can 3D print a longer magazine. Something technically illegal in my state, but, if I go travel to a nearby state and purchase an even longer magazine than I plan to print. Well, that's entirely legal to take back into the state. Eh, why risk it?

So, again, it's more about profit protection than it is stopping someone that is malicious. It's another "step" that makes them actually consider the consequences of not giving money directly to weapons manufacturers.

It's about adding enough "risk" on paper, to the citizen that wants to just print something, that they instead just purchase it (either from another state) or locally. At least I'd say that's the primary purpose and outcome for this law specifically.