Even at 20th level, a wizard can't afford to toss 6-8 high level spells per encounter. Of course, at 20th level, 6th level spells aren't high level anymore.
That would be 5 high-level spells in a 5 round encounter (possibly 6 - if you use
improved invisibility right before combat starts.
1. That the multitude of "I want long, epic, knock down drag out battles, that's what's really fun" people get their way. In that case, 5 rounds wouldn't be a typical encounter anymore. (Of course, if the typical encounter right now is 5 rounds, I don't see what they're complainging about--IME the typical encounter is more like 2 rounds (and that's with 28 point buy, and 1/2 DMG gp value PCs)--five round battles seem pretty epic knock-down drag out affairs to me).
According to the DMG, 5 rounds is supposed to be the average encounter length.
Not two rounds. As a result, 5 rounds isn't "long, epick knock down drag out battles". Something like that 7 round encounter I had two weeks ago (against a bunch of wargs - I was 2nd-level) is more like it
This two round-phenomenon is an artefact of weak high-level encounters. It's rare that a 15th-level fighter is really a CR 15, for instance, and most of the high-CR monsters have a glass jaw somewhere (low AC, a low save, etc) that means they drop out in round one to three.
2. The use of 3.0 Haste or Quicken spell.
Ever wonder why some people hated
haste?
And, to my mind, it reveals why save or die spells are balanced--and even exciting: it's quite possible to go a number of rounds casting two spells a round before the opponent sees any effect from the spells at all.
If only if were that way. Sure, it
could happen, but it doesn't seem to do so that often. If this happened more often (not all the time) then it would be more fun, and it would give the fighter a better chance.
That's the risk you take when using save or dies (which seem (at least from 20th level wizard vs. Pit Fiend or Titan examples) to have at best a 25%-30% chance of success.
Most of the high-CR creatures in the MM 3.0 didn't have these kinds of defenses - except dragons.
When players want to gamble like that, I don't see any reason to complain if they occasionally win.
They're always supposed to win

However, they're not supopsed to "often" end the battle in round one, they're supposed to "occasionally" end the battle on round one.
Actually, I don't think high AC would do much about the things that one-hit kill monsters. Even the Keen Improved Crit Scythe only crits on an 18, 19, or 20 for instance. Even with good AC scores (40+ range) those kind of rolls will hit on 20th level fighters' secondary and tertiary attacks. (Confirming may be another matter but even counting the need to confirm your standard 20th level fighter (28 strength, GWF, +5 scythe) PC has a 28% chance each full attack to score at least one crit with his keen imp crit scythe against AC 39)
There was a rumor at 3rdedition that keen might be altered. Alas, it isn't as substantiated as Shadowstar's news.
More importantly, I'd like to see a 21st-level fighter with a
+5 lawful flaming burst scythe do 370 points of damage in oune round. Lots of high CR opponents didn't have enough hit points, just like lots of high CR opponents didn't have good enough saving throws.
We probably hear more complaints about spells than Improved Crit because the spell is more likely to knock out an opponent, regardless of it's hit points, and because a spellcaster that can knock out an opponent 50% of the time is a lot more likely to do that than a fighter who can knock out an opponent 28% of the time.
*Of course, you don't need to be as lucky to succeed with save or die spells as you do with high crit multiplier weapons. Often your chance of success is better than 50%. On the other hand, you only get a few shots at success with the spells while the weapon users can have up to 4 (or 8 if they dual wield picks) shots per round.
This is the meat of the matter. I haven't seen any high-level fighters using a scythe, so I hadn't had this happen to me. However, as you've pointed out, this is less likely to occur than with a save-or-die spell.
And if the fighter doesn't get lucky, he's getting pounded, putting him in worse shape before they face the BBEG. (Or maybe the cleric is using up spells healing him.) Either way, resources are being used.
(The exception is a vorpal scimitar, a cheesy weapon that is about to get a well-deserved nerf.)
PS I had a "brain fart" over
remove paralysis, but there's still no
disintegrate ward spell.