Yes, in actual play, I have seen modestly optimized AC gishes become unhittable, and seen DMs boost monster ATK values in later encounters to compensate, making lower AC characters become auto-hit and modest AC characters almost always get hit.Question: Have you found this to be an issue in actual play, or is this just a concern about future play?
The AC bonus is admittedly a bit large, but it essentially makes attacks against the character one DC category higher if we're looking at it in the bounded accuracy math.
OK I gotcha now.Yes, in actual play, I have seen modestly optimized AC gishes become unhittable, and seen DMs boost monster ATK values in later encounters to compensate, making lower AC characters become auto-hit and modest AC characters almost always get hit.
That was describing actual play experience.
I have seen people, including in this thread, advise people DMs to do exactly that (boost monster ATK until the high-AC character is hittable). So this isn't an aberration. I have seen DMs say they do just that.
Hell, AC-arms-race is something I've seen in most versions of D&D I have played! It doesn't happen in every game of each edition I've played, but it happens.
When someone finds a way to have AC so high that even tough monsters miss, magically the monsters to-hit abilities scale in return. And AC stops being "monsters hurt you less" and instead becomes "monsters hurt your allies more". 5e has the distinction that they addressed this issue head on -- that is one of the components of "bounded accuracy" -- and they did a pretty good job, so much so that I suspect fixing just the shield spell might be enough to make the problem go away.
It is very confusing to me that this common problem in D&D is being met with incredulity, as if "what, how could that ever happen?!".
Yes.OK I gotcha now.
For me I would just max out the AC you can get with the Shield spell at 25. But I can see if you don't want to do that, then your solution of just applying to one attack seems right. To scale, just let it apply to one more attack for each 2 levels above 1? So applies to 2 attacks for a 3rd level slot, 3 attacks for 5th level slot, 4 attacks for a 7th level slot, and 5 attacks for a 9th level slot?
I guess that isn't too bad wording wise. I was worried adding extra "stacks" would make at-higher levels suck. But it doesn't seem to.An invisible barrier of magical force appears and protects you from attacks and magic missiles.
While this barrier is active you gain a +5 bonus to AC, have resistance against damage from all attacks, and are immune to damage from magic missile.
The barrier lasts until the triggering attack, or magic missile spell, finishes.
At higher levels: For every spell slot level higher than 1st used, the barrier lasts for up to one additional attack or cast of magic missile that targets you, or until the start of your next turn, whichever comes first.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.