There is not an inherent flaw in having a house reserved for those who value or demonstrate bravery above all else.
What about those who seem to demonstrate characteristics other than bravery?
As you point out in your question, many have questioned why people such as <PERSON>, <PERSON>, <PERSON> or <PERSON> and <PERSON> ended up in Gryffindor, when they demonstrate other features such as cunning, loyalty or intelligence. I believe these are adequately answered in the questions you link to, but the reasons, briefly, are:
* The Sorting Hat realizes their potential, rather than their current state (<PERSON> certainly had the potential to be brave, but hadn't necessarily demonstrated it)
* They're smart, but they're not into learning for learning's sake (as demonstrated by <PERSON> and <PERSON>)
* They valued bravery more than intelligence, for example:
"Me!" said <PERSON>. "Books! And cleverness! There are more important things – friendship and bravery and – oh <PERSON> – be careful!" - Harry Potter and the Philosopher's Stone
"Ultimately, she had a bigger heart than she had a brain, and that's saying something for <PERSON>." - <PERSON> on <PERSON>
What about brave people in other houses?
Just as members of Gryffindor present intelligence, cunning and loyalty, members of other houses can present bravery. Because it goes both ways, I don't think that Gryffindor is any more redundant than the other houses.
* <PERSON> is brave as hell, but is better known for her thirst for new (sometimes slightly squiffy) knowledge and natural curiosity.
* <PERSON> was brave, sure, but also side-switching, manipulative and drawn to Death Eaters and dark magic. He lashed out at <PERSON> when she tried to defend him, and was less-than-chivalrous during his teaching (trying to snitch on <PERSON>, becoming <PERSON>'s <PERSON>, etc).
* <PERSON> risked her life protecting her son, but also would only have done so for her son, not anyone else. She displayed no intentions to protect anyone beyond her loved ones during the books.
They all show individual bravery, but ultimately, are not characterised by it. 194
By contrast, <PERSON>'s, <PERSON>'s, <PERSON>'s (etc.) key features are their bravery. <PERSON> is never particularly portrayed as cunning or academic, <PERSON> steps up whenever it really matters, and <PERSON> is far more at home among books but still decides to dedicate her time to helping the Boy Who Lived, because she thinks it's the right thing to do. Importantly, they do so every single time. So yes, I'd say that the Gryffindors are really more brave in that respect.
You mention members of the other houses who join the Order, or help during the Battle of Hogwarts. This is, no doubt, a brave thing to do. However, in the midst of battle, knowing what's at stake, it isn't surprising that they would choose to do so - they might just consider it a bit more carefully than the <PERSON>. And, again, it's not their defining action. 190