I think that's the point. Agile/Scrum is often just window dressing. If you have a good team then it will give you some nice things like frequent interim releases and a better idea of progress. But if you don't have staff who are interested, disciplined (and talented!) then you're screwed either way. And I've met a lot of staff who are not like this.
Frankly I'd rather that people had a mandatory five-year term in a disciplined team at one of the traditional, enterprise-class, quality focussed firms that demand good practice. I can work in whatever methodology you want, but that made me the software engineer I am today.
It has its dark corners like anywhere else, I'm willing to concede it may just have been the teams I worked with that were awesome, it's certainly not cool, I wouldn't want to spend my whole career working for them and (frankly) god help you if you let one of their salesman in the building... But I learned a lot about doing things right. And now I'm a freelancer without the massive organisational overhead so I get to do things fast and right.
I dunno, maybe a large traditional corp isn't the only answer, but some way of enforcing good practice on people for a while, like a boot camp, so that they can take some of it with them to environments where it's not enforced as rigidly.
That's pretty much the #1 reason I find Scrum useful. It makes shit hurt. When introduced in a disfunctional organisation, it's basically a role-playing version of root cause analysis.
Of course, when implemented internally in a disfunctional organisation by people who are part of the problem, those issues will be swept under the rug and the problem will only get worse.
If an organisation doesn't want to change (including making the hard decision of firing the incompetent and undisciplined), not methodology will help.
And neither will your manifesto.
Yes, everything depends on having the right people and having them organise themselves (which is the very foundation of Scrum and most Agile methodologies). But in order to organise yourself it's helpful to use a clearly defined method instead of re-inventing the wheel.
Teams that fail with any Agile method are likely to fail at self-organising anyway.
No methodology solves the people problem, but some methods, particularly Scrum, are very good at identifying the exact nature of the people problem.