if i could give my own thoughts on this, that's aside from trading woes...
...the shinies is like this extra thing that adds further complications and extra goals to the game when there are enough already going around
goals:
competitive deck building (re-adjusting to meta changes)
fun deck building
card/collection completion
daily, weekly, season, long-term achievements
getting top in league
collecting themes and sleeves and avatar
there is also a one-card collector hobby (people who hoard dupes for the sake of dupes) that is like the shinies but at least there is no real official incentive there
in short there is a lot already to think about, the shinies don't add excitement tbh
but i get it, it's a business more than anything, you guys need to find ways to sustain it, and you can't keep churning out cards quicker than you already do even if technically this game is called Cards, Universe, and Everything, because it will make balancing difficult and will easily overwhelm players
to me the shiny should be something that's based on chance and not through fusing and trading and nothing more (like a real thing to brag about look at me i pulled a shiny)...
now whether there would be packs dedicated to have a very small chance of pulling a shiny that could be the alternative way to earn money i'd be fine with it