The “they’ll get the guns anyway even if illegally” argument is nonsense. By that logic, why bother having laws in the first place?
In many states, as well as federally, smoking pot is illegal. People still do it constantly because they have some sort of access to it illegally. The same applies to pretty much any other product. If there is demand for it, at least a few of those demanding it will get their hands on it somehow.
Any other illegal drug, alcoholic beverages during prohibition, alcoholic beverages to minors today, porn to minors, child porn, etc. is accessible in some way to those who seek it hard enough. The same applies to weapons, especially considering the whole 3D printing situation.