Ok, after giving some time to cool down things here's my doubts about your arguments:
-You claim "lowest Servant ability is ten times the normal human maximum and progresses linearly"
-Gilgamesh's game combat stats Strength: B, Agility: B, Endurance: B
-Gilgamesh's performace against a sleep deprived modern day teenager assuming both have similar weapons- Gilgamesh gets arm chopped off and clinging for his dear life. If student is willing to sacrifice himself, Gilgamesh is toast (yes, shirou can solo gilgamesh whitout Archer's help if you don't mind geting a bad end).
-Shirou isn't at human maximum. Gilgamesh doesn't have the lowest servant abilities. It shouldn't be anywhere close. Gilgamesh should be able to turn Shirou into a blood puddle before he realizes what's happening with his bare hands. yet Shirou keeps up with Gilgamesh and drives him against a
wall infernal pit.
Point being, you can't claim something is dozens of times better than another thing, when those things are ending up pretty close. In the two first routes Shirou again and again evades/survives direct servant attacks. Even Lancer's suposed insta-kill lance just leaves him bleeding out, allowing for Rin to get some healing magic on him.
Speaking of which, berseker can actually tank at least one excalibur hit (first route bad end if you don't let Shirou help in the hacking). Rin meanwhile suceeds in blasting one life out of Berseker. Meaning you get twelve mages of the level of Rin (and she's a student) and Berseker can theoretically be defeated whitout any servant help.
See where I'm going? If servants have such high power, how come regular mortals are still so relevant? If there was so much diference of power as you claim, then mortals should be nothing less than irrelevant, and any master who shows his face in the open away from his servant would be reduced to a smear of blood before being able to spend a command for help.
Also, I wasn't really using optimized characters at all on my examples of high level D&D. Time Stop. Meteor Swarm. Astral Projection. Gate. Shapechange. Teleportation Circle. True Ressurection, Elemental Swarm. Those are the
basic 9th level spells of the caster lists. They're not combos or obscure tricks, they're the normal (as you put it) a caster is expected to have once they hit those levels. And they're all beyond what anybody in Fate/Stay Night can come close. Maybe an honorable mention to Gilgamesh and his uber nuke. But then Gilgamesh is being humiliated in melee by regular teenagers, when he should have more than enough HP and Bab to just stand there and let himself be hit by Shirour whitout problem if they have as much level diference as you claim.
This is, Caster couldn't even set up a
clone. Again, not an extreme. Just basic 8th level magic.
Anyway, high level D&D characters are anything but normal. They fall of mountain cliffs and get up. They swim in lava. You can eat them and chew them up and they'll just chop their way out of your stomach. Really high level D&D stuff goes around murdering/changing whole worlds (yes, multiple). Servants, meanwhile are fighting in a single town in a scale where basic mortals are still extremely relevant. Yes they save the world from some ancient horror, but, again, that's something that fits in mid-levels D&D.
Just think about it. Geting killed in Fate/Stay Night is a pretty big deal. In high level D&D it's a minor incovenience.