Here's the thing about shiny variants...they are rare. Intentionally rare.
Because they are so rare they have a special 'bonus' of their leveling up 'cost' being discounted.
The 'street cred' of having a shiny in your collection means you had a very lucky pull. Reaching Lv 100 prestige, for any rarity, is statically easier than pulling any shiny. So being able to unlock a shiny variant undermines the scarcity/specialness of pulling a shiny variant.
I mean this is a gatcha-game, it is by design that if a player wants to chase shiny variants, all they have to do is open (a lot) more relics...which in turn requires more Theo...
That said...
Do I think that if you max out a fighter's prestige AND XP AND Marquee levels that there should be some unique 'visual enhancement' to signify that achievement? Yes, but it is not unlocking shiny status.