If your AC is already advantageous, then buffing it for a round isn't going to be a substantial help. One of the things that makes shield so good for the unarmored caster is the caster's initially disadvantageous AC. Additionally, I call foul on the Action Surge bit; you're assuming the presence of a resource that may well have been exhausted earlier. Even if we assume that both parties have access to their action surges, the increased AC from shield is effectively negated by the advantage the full fighter could get from first shoving her enemy prone.
So you're saying that if your AC is good enough that you're only hit 70% of the time, then dropping that hit rate to 45% isn't advantageous? That doesn't track.
Sure, if you've boosted your AC into the mid 20s and you're attacked by goblins with a +4 to hit, Shield is pointless. Against an Ancient Dragon with a +15 to hit (or other powerful enemy) there is very much a point to it.
No need to call foul on Action Surge. Ignore it if you like. The Fighter 20 still has 4 attacks with a 45% chance to hit versus the Fighter 19/Wizard 1's 3 attacks with a 70% chance to hit. Still clearly advantageous. The F20 may very well still win the fight, but Shield gives the F/W a solid fighting chance. That's all I was trying to show. Remember, I agreed that the F20 is superior in terms of raw power. I'm merely showing how the F/W is 20th level in capability. Breadth vs depth.
My turn to call foul. If the F20 can knock the F/W prone, then the F/W can knock the F20 prone. Advantage gives the F/W a 91% chance to hit, whereas it only give the F20 a 69.75% chance to hit.
Additionally, expeditious retreat is a concentration spell. Sure, the fighter likely has no other spells she needs to concentrate on, but she also most likely has to put herself in harm's way in order to hurt her enemies, and that means the concentration is likely to break quickly even with the fighter's Con save proficiency.
So? The F/W is only using Expeditious Retreat to engage with the enemy so that he doesn't lose his first round of attacks. He's not likely to lose concentration when he casts it, since it's on his turn. If he maintains concentration, great, that's icing on the cake. If not, it's already done its job.
In some cases, it is a matter of being able to participate. It's hard to participate in a social encounter if you don't speak the language. Comprehend languages then allows participation. It's hard to participate in a social encounter when there is racial or economic prejudice afoot. Magically disguising oneself as the proper ethnicity or social class allows participation.
Sure, in some cases. And in some cases, like an anti-magic zone, having spells might not help at all. But there's a lot more to those utility spells than a simple pass/fail on being able to participate.
Except that it's not advantageous; it's another just means of participating because the disguised fighter still has to get the disguise right. All you're doing is shifting which kind of check you are making.
As I said, if you use Disguise Self and succeed, then the target doesn't know that you know. If you bribe or intimidate him, then he knows that you know. Knowledge is power, therefore it's reasonable that it's advantageous that he not know that you know.
Yes, that's why I said feather fall was good. Congratulations, one spell out of an entire level proves advantages across the board (you know, assuming that board takes place above ground level).
I've shown how a number of spells are advantageous. And there are many more I could demonstrate the advantage of having access to including: Alarm, Charm Person, Detect Magic, Find Familiar, Fog Cloud, Grease, Identify, Illusory Script, Jump, Longstrider, Protection from Good and Evil, Silent Image, Tenser's Floating Disk, and Unseen Servant. That doesn't include the wealth of cantrips that can be useful to even a 20th level adventurer, nor does it consider expanded spells from things like the Elemental Evil Player's Guide.