My point is that if you have to face 5 fire giants or more at Level 12 then its a TPK. In a fortress of 105 fire giants
I partially agree with you on this but I think you have it slightly back to front (at least based on my observations).
The CR system begins to break down at higher levels primarily because the monsters (especially those from the Monster Manual) do not match the statistics for monsters of their supposed Challenge Rating (as per Page 274 in the DMG).
Now yes more options, items and spells CAN result in overpowered Party's but if you don't get the monster basics right you just compound any potential problems.
Githyanki Knight ( CR 8 ): 91 hp and two attacks averaging 46 damage.
CR 8 (by the DMG): 176-190 hp and averaging 51-56 damage.
Fire Giant ( CR 9 ): 162 hp and two attacks averaging 56 damage.
CR 9 (by the DMG): 191-205 hp and averaging 57-62 damage.
So the Githyanki Knight has 49% of the suggested HP for CR 8 and 86% of the damage. Githyanki Knight at 67% overall effectiveness.
A Fire Giant has 88% of the suggested HP and deals 94% of the suggested damage. Fire Giant at 91% overall effectiveness.
Both have fractionally higher than average AC's so we can cut them a tiny bit of slack. But we can easily see which one is more formidable on a per CR basis.
I think the MM fire giant has better to-hit as well as better AC, compared to the DMG CR 9 baseline?
If anything I'd say the MM giants are among the few MM creatures that do feel worth their CR. Many monsters feel terribly underpowered.