Two silvers alone probably wouldn't guarantee a selection to the world team, unless no other teams had any international medals at all in the past year.Say a team earned two silver medals at easy grand prix's, why go to nationals? You get worlds based on BOW.
If they're skating well and expecting to be in contention for a Worlds spot, they probably also expect to be in contention for a national title. That would be incentive enough for most to want to compete.Teams that did well during the season don't have to risk nationals because they get worlds anyway.
Of course if they're ill or injured in January and don't expect to skate as well at Nationals as they did earlier in the season or expect to in March, or their skates broke the day before the short program, that might be a reason to skip Nationals.