I've been thinking on this. I think this dilemma is resolveable without SE having to change a lot. It seems the execution of SE's goals was: group content for spells, solo content for Carnivale. I think they can embrace this distinction and give longer lasting BLU content that's more interesting than leeching and gives Carnivale more longevity.
  • Spells from level 50 content cannot be obtained if undersized
  • The drop rate of spells for level 50 content is increased (especially for primals, they will definitely need a big boost). EX should give a higher drop rate than HM. So there's an incentive for the EX fights over HM.
  • Add new Duty Finder options specific to Limited Jobs - a roulette or DF option for the level 50 dungeons with spells in them, Primals will need to be PF'd.
  • Add a new tier to Masked Carnivale, have fights that require spells from level 50 content (if they have limited resources for this, hard mode versions of the existing ones). Give rewards useful for level 70 main jobs. This tier would pose bigger challenges to you.
  • To complete this tier 100% you will need to have collected all level 50 Blue Mage spells. You get a special achievement & reward for completion

I think this would do the following:
  • Open more content to Blue Mages. Rather than have them leech, which is boring.
  • Offer something to those who want to do group content on BLU and have it feel like content.
  • Make the Solo Content the end focus. Have it be challenging and not lose relevent after a couple of hours. Give more incentive to repeat fights. And completing it 100% will take time and offer a sense of achievement at the end
  • Give extra love to old content, those who're nostalgic or joined the game too late can enjoy it. ARR had some great endgame content.