Personally, I wouldn't care if mute swans were a rarity. There should be a shooting seasion for them - just think of the drumsticks!
It was significant back in the '80s when I wrote a lot about lead poisoning of swans that the majority of problems (and lead weights DO poison swans) were on navigable waterways.
I spoke to one scientist at the RSPB who categorically told me that if the wash from boats hadn't silted the gravels so badly, swans could find all the small stones they needed. On silt, the only small, hard objects in the upper layers were discarded shot, below, the gravel the swans sought.
He also commented that he had never seen a lead-poisoned swan on a gravel pit. But of course when I asked him for this on the record, he told me his evidence was unscientific.
It is undoubtedly daft to go spreading lead through the countryside willy nilly, but we aren't any more. The swan lovers had their day, and should concentrate their efforts on something important, like the fate of rarer birds.