The point is to get guns off the street. Grandpa’s gun that’s rusting out in the barn isn’t really a concern, but trying to write a law that (pardon the pun) grandfathers it in invites loopholes that could be exploited by bad actors.
So, instead, say it’s illegal to own Gun X. If you own Gun X, you can turn it into the police, and they’ll buy it back. The deal is open ended, and never expires. If you don’t take advantage of the buyback offer, there’s no specific gun enforcement division that will go kicking in doors looking for illegal guns. But if a cop finds a banned gun, it gets confiscated, and you don’t get any money for it. Maybe slap a small fine on top, or (if there’s other charges, like robbery or assault or whatever) there’s an enhanced penalty for using a banned fire arm.
So, some (admittedly small) percentage of the guns will be taken off the streets through voluntary surrender. An increasing percentage will be gradually taken off the streets through the normal enforcement of other criminal laws. And a diminishing percentage will stay hidden in the collections of otherwise law abiding gun owners, until they eventually die out and their collections are inherited by people who will either A) turn them in to comply with the law, or B) get them confiscated because they’re doing other illegal stuff, and the cops find the guns.
It’s a slow plan - it’d take decades to actually eliminate any particular type of gun this way. But it’s a way to do it that doesn’t turn a bunch of otherwise law abiding gun owners into felons overnight.
Most other “gun controllers” would, I think, be fine with this plan, because the point of gun control is to reduce the amount of gun violence in society, not to just be a dick to people who like guns.