The game is balanced around 6-8 encounters but casters have enough spells for 9 or more encounters depending on DM difficulty, then it takes more long fights to grind down their spells in order to reach parity down to half casters and noncastets.The bolded is where you said it. It is implied. I have to chip the casters down or else...
And I am telling you that is not even remotely close to true.
You also said this:
You are saying the game is broken at high levels. In your own words, your very first reason is: "The DM has to burn down a lot of spell slots in unrealistic ways in order to create a challenge..."
And I am telling you that is 100% false. Wrong. Not correct.
Now, if you were just talking about the length of time it takes for some casters to take their turn, I am 100% on board. I think you are correct. But it is patently wrong to say the DM can't challenge them.
A number of encounters that is hard to get to because caster turns are so long their contribution to fatigue is massive and some high level spells provide options to shorten ones day.
And soon noncaster turns will be long as well via masteries and new class features.