(Note: Most of the following is a perspective moreso than a counter argument on my part.)
Kyurem really wasn't Gliscor's main counter all things considered if you get past the "Ice into 4x Weak Ice target" idea.
Kyurem has very few safe entries into Gliscor without a Pivot (several of which can't do much against Gliscor either) considering how much it hates the mon's typical Progress Making tools (Knock Off, Toxic, Spikes stacking) or just stray attacks (30% from EQ or Poisoned Facade isn't negligible on a mon that needs Leftovers for recovery), it's only a dangerous or winning interaction once it's on the field, but getting there isn't really free. Even with as busted as Kyurem is, a team using it as their primary Gliscor contingency wasn't well constructed. At the same time, Gliscor doesn't answer Kyurem directly either so it's not like people are compelled to keep it in the tier to answer that in turn.
I'd go as far as to argue that Kyurem being around makes Gliscor even harder to deal with. When your team has to account for a powerful mixed attacker with several Balance breaker options, it's tricky to also account for the free-progress Scorpion that can runaway with momentum off even a hint of passivity and recovering 25% health just for existing for 2 turns. Both mons in different ways absolutely choke your space for teambuilding to a very cramped design space if you want to fully account for them, so it's entirely possible that us being stuck with Kyurem for the time being will make people MORE inclined to remove Gliscor as an extreme constraint on team structures.