Why are you speaking in such absolutes? There's nothing incompatible with determinism and RNG. Clearly D&D plays with that continuum everywhere. Having consistent advantage does not negate all challenge nor remove player agency from the game. That's hyperbolic and flatly wrong.
It literally does. Every single aspect of D&D 5e, from ability scores, to HP/AC values, monster abilities, class and racial abilities, spells, pretty much every mechanic in the entire rulebook is balanced around the player *not* having consistent advantage. How do I know? Because there's a 9th level spell, the most powerful *thing* in the entire game, that does nothing else beyond granting Advantage and imposing Disadvantage on enemy attacks. That's *all* it does. And Lazy Larian now lets us replicate 50% of that spell (only advantage on attacks) just by dropping a box and standing on top of it.
Even though advantage only comes to be around a 5% bonus on average aka a +1 static bonus? Even though just because a spell has a given slot means it can either be very weak or very strong in that slot? Even though advantage or disadvantage can be ridiculously easy to negate?