I think there are definitely better ways of identifying skaters who have proven themselves as not needing to qualify than using the results of one competition that took place a year earlier.
Top X in the ISU standings as of the close of entries would be one way to do it.
Another way would be to exempt that year's Grand Prix finalists . . . after all, they already had to do an extra competition that year and proved themselves to have been skating well enough in the fall season to qualify for the GPF. Also *medalists* (or possibly top 5 or top 6) from Europeans and Four Continents, which would have been the most recent major senior competitions . . . and would give an incentive for medal-worthy skaters to attend those competitions, esp. 4Cs . . . sure it's more traveling and competing than if they don't go, but they'd save themselves from skating an extra program at Worlds.
Junior Worlds fields are often larger than at Worlds, so qual rounds and potential exemptions would also be a consideration there. For exemptions from qual rounds, I would suggest JGP finalists from the current season and any returning *medalists* from the previous year's Junior Worlds.
If there were anyone who qualified for the senior GP final and then returned to Junior Worlds, I would also exempt them . . . but the only examples I can think of offhand are Mao Asada 2006 and Caroline Zhang 2008, and in both cases they were also (gold) medalists from the previous year.