But all the GWM analysis does include the chance of missing.
You have totally missed the point, so far over your head the breeze didn't even touch your scalp.
Look at every GWM analysis. The most common is GWM with BM precision strike. As an example none of them compare that to a Shield Master fighter using BM Precision Strike. The Shield Master will always hit more, as under the same conditions the Shield Master will have +5 to hit and +2 to damage from Dueling. The base comparison should be -5/+10 against 0/+2 (for dueling style) So we start there, and then add in precision strike.
The Shield Master will also be alive longer to do damage, certainly more then just the +2 to AC would indicate as you do get some save bonuses also.
But we are not done, the Shield Master fighter will have mores instances of advantage because his bonus action will be used to prone people, creating more advantage for himself. How much? Well you it depends, but from experience more than you think, as you fight many enemies size large or smaller. I would posit about %25-35 more instances of advantage but its just an observation, I don't track it, you also get 2 chances at a crit. When you have a player with Shield Mastery in your group it also makes Enlarge a useful spell or potion or scroll or magic item. The Shield master will also create advantage for his teammates, which a GWM fighter doesn't do. When the Shield prones someone and then the rogue or paladin lands sneak attack or smite some of that damage is because of the Shield Master, especially if their hit is a critical hit. Setting a Paladin up for a double smite is a big thing, but not really quantifiable.
Finally a shield master adds damage by pushing people off cliffs, ledges, into lava, into a Spirit Guardians zone and then damaging them twice (6d8 total, 2 saves + speed reduction), once on the Shield's turn and also when the creature starts its turn there.
A raging Barbarian with a Shield Mastery gets advantage on all those checks to prone people, they succeed even more.
As a note, Shield Mastery is used more, if you don't have anything else to do with your bonus action you take a shot, and its used on defense also. GWM sometimes you turn it off. Its just fun to use also, more than just getting bigger damage numbers.
Here on the forums in math rooms, the GWM will always win any static DPM challenge since many things you can do with a Shield Mastery feat are not quantifiable. At the tables were the important number is effective DPR on a target for the whole team, its a wash at best. The GWM is good in every group and can always be useful at what they do, but a Shield Master will be able to do more and different things and will get alot more of the groups damage across.
The point is that in evaluation these builds and how feats interact with them the analysis is always short, due to lack of information about how it is played at the tables. While some feats are certainly better than others, I don't think of any of them as out right OP. IMO Bless spell is more OP than any feat is.