Registering guns is just so you can later ban them when you get the political will...
No, it isn't. It's about collecting information so that one can use that information to subsequently take action against people who betray the trust placed in them when they chose to exercise their right to own a firearm.
And no, owners of black market guns are not going to register their guns. But by having stricter rules that allow for the easy tracking and verifying the flow of any given gun from lawful seller/owner to lawful owner/sell to, eventually, an unlawful owner tells us who is responsible for failing to exercise adequate control over their weapon.
The problem is not that lawful users obtain guns; it's that unlawful users do. The only way to find out what ostensibly lawful consumers are/have lapsed in their duty to make sure nobody "untoward" gets hold of their gun, is to require folks to document/attest to how and when they came by their gun and how and when they yielded/lost possession of it.
For example, if you buy a couple guns and register both of them, great. Enjoy your weapons. If one of them gets stolen and you don't report the theft, there comes about the first "building block" in a plausible argument that you (1) were negligent in exercising due care over securing your weapon, (2) conspired to allow your gun to be stolen. Now one theft is not really indicative of either, so the presumption of innocence still accrues to you. In time, you replace you stolen weapon and buy several others and register them. If they don't keep getting stolen, there's little cause to think you are routinely negligent or willfully part of a supply chain for getting legal guns into the hands of illegal users.