The reaction I expect is "interesting, how often and how easy is those mitigating factors to come about?"
Since I know the answer to that question is "Depends on the DM, the players, and the campaign they are playing", I don't see why you expect me to be asking it.
The designer of the feat failed to realize how many and varied the ways are to make the -5 part go away, either in full or in part. This in turn makes the feat useful in too many situations, against too many monsters.
Alternatively, the designer of the feat simply had a different idea of what the typical campaign played by a typical group of players with a typical DM is like than you do. Or possibly even just thought "This will work as I intend for the folks not heavily focused on the numbers and optimization of things, and those folks that are heavily focused on the numbers and optimization will be able to see that this option is out of line for their games and make their choice about using it as-is accordingly." You know, like has happened - except the part where you also insist the feat doesn't work for any of the rest of us because we haven't leveled up yet.
...as in there is no other combat feat providing even remotely the same bonus.
And I think that in such a game the duel wielder feat could easily be competing for damage, since I can do as you have done and assume other factors beside the feat such as that the character is wielding two potent magical weapons. Which I say not to argue that you are wrong about the scenario, but that the scenario shouldn't be considered in the way you are considering it - by which I mean a reasonable conclusion from the evidence would be "I shouldn't pile all these options into the same campaign because of what happens when they are used in combination", while "That feat is broken" is not a reasonable conclusion from the same evidence.
Now we're definitely leaving the kindergarten behind.
It is rude to continue to suggest that to disagree with you or play the game differently is to be less learned.
The penalty reduced to -1...
It undercuts your point to phrase things this way, as the feat's penalty is never reduced. It remains -5 regardless of what other factors apply to improve accuracy, since those accuracy improvements apply even when not taking the -5 penalty.
As for the Strength 22, you might be ready to decry, say a Belt of Giant Strength as the real culprit. But again, no, the real problem is the feat.
No, the real problem
is the combination. The combination is not an inherent and unavoidable trait of the feat, so the feat (or any other singular piece of the combo) should not be blamed for the combination.
And I am really confused why you pick the feat to blame when you have said that it isn't unbalanced by itself, and even said this:
Yes, the belt adds +1 +1 on top of what's normally possible, but that is in itself not terribly unbalancing. It is the compound effect...
Telling me that the reason I say the feat isn't broken does not apply to the feat, but does apply to a magical belt.
That is still way higher than the designer must have foreseen.
There is no "must" there. The designer could be thinking the same why I do, that combos are allowed to be crazy powerful because it requires actually setting them up and knowing when to use them to really make them shine.
That does not mean that those "ways" (bonuses from spells, items and class features) are themselves unbalanced.
Again, you say those other elements are excused from being considered broken because they are involved in a combo that produces results you think are unintended - but give no reason why the feat is not being treated the same.
Let me adress the "what other martial character" bit:
I am saying the feats' biggest issue is that it steers players towards build choices involving greatweapons and ranged weapons, because that is the only two weapon groups that can be used with -5+10.
I have seen no such steering among my players. Might you have meant a particular sort of player rather than literally all players?
Just because it no longer is an issue in your campaign does not mean it ceases to be a general problem.
Just as it being an issue in your campaign does not mean it is a general problem.
But when you add feats, all of this is thrown out of whack. Because suddenly one character (the barbarian in this example) will start doing damage leaps and bounds above the other one.
I find that not to be the case. My campaign with two fighters in it (one eldritch knight and one battle master) has one that took Great Weapon Master and one that hasn't, and nothing is anywhere near "out of whack."
For a player with at least a nominal interest in the "game" part of the session, that is, a player not oblivious to his character's effectiveness, it no longer is excusable to abstain all that damage.
Implying that people of different opinion than you aren't really interested in the game or are oblivous to their character's effectiveness is rude. It's also incorrect.
So asking questions about exactly "what person is this" is completely missing the point. The answer is simply "anybody that's trying to be a fightery character but not taking those feats will regret that choice if playing in the same group as somebody that did take those feats"
Pointing out that not all groups end up with more than one character that might want one of these feats is far from missing the point - it's a point of its own; not all player groups out in the world playing D&D are concerned with things like a team-mate doing better damage than they are (because they spent a limited resource on a thing that only does more damage, instead of on something else that may apply to a wider variety of purposes).