legalised theft is all it is. If it applies to firearms what else does it apply to?
Two ends of the precedent spectrum come to mind for items that have fallen under the same/similar police powers.
On the intrusive, imposing financial harm end is is asbestos in homes. Government action not only banned its use, but hung a future liability on property owners (in the form of required abatement if disturbed/discovered) that had purchased what was (at the time of purchase) homes made with legally conforming building materials.
On the non-intrusive - incandescent light bulbs. Banned from commercial sale to stop their proliferation, but no effort to stop their use if they were currently in use or storage by homeowners.
Applying the lightbulb method to firearms - no longer available for commercial sale, but private owners still have full use and ability to transact in the secondary market
Applying the asbestos method - no longer available for commercial sale but private owners, still have full use while the good remains in good working order. No replacement parts available for commercial sale forcing premature end of life, and end of life requires owner foot the bill for safe decommissioning.
Both vastly more justifiable than criminalizing use of lawfully obtained property and forcing it out of people's hands. They may not have a legal responsibility to compensate, or compensate fairly- but they sure as hell have a moral one. Objective best practice is to grandfather, and if they need/want change faster then that objective best practice is to pay up. They were they're conducting this is self-righteous and cynical bullying of the "other".