First, could your experiences with sx/soc 6s be environmental at all? Like you indicated earlier that you lived in a crap neighborhood with lots of problems, so if many of the 6w7s you know came from that area, it's possible there are other factors at play here.
Second, I'm pretty sure all sx/soc 6w7s aren't failures and whatnot. Not anymore than all sx/soc 8w7s are all dictators.
The only example I can give is that of my father. He's probably more of a 5-winger than a 7-winger (I could see an argument for the 7, though) and is sx/soc, or possibly soc/sx (I can't really tell). And he's got a lot of neurotic problems, but being a drug-addicted fuck up is NOT one of them. He's a career-minded individual. He knew from the time he was in college that he wanted to be a professor, and has spent most of his 67 years building that. He has issues around success like any 6 (he doesn't want to be attacked for being "successful") but he equates failure with disaster. Most of his moves in life have, in his own words, revolved around "staving off disaster". At age 67, he owns and manages his own property and has learned to eat a healthier diet. He's concerned with acquiring a better salary and making sure I have the material goods necessary to support myself. His care for sp stuff isn't great (he gets annoyed with "housekeeping", insists he'd rather die young than live till 90 in a nursing home, and has heart and pancreas trouble from a lifetime of living off junk) but he "gets it" that money and stuff is important.
And yes, he's 3-fixed. 683, I'd say.
So, I don't see why being a sx/soc 6w7 is necessarily conducive to failure and not being success-oriented. It sounds like the 6s you know are failures because they've got other issues, not because they're sx/soc 6w7s. I don't personally claim to know your stacking, Great One, as I don't know you IRL. But, I think you should assess it by your motivations first, rather than by comparison with crappy people. Each individual is a unique permutation of his/her type, stacking, and MBTI and you may do things contrary to what you'd typically expect to see. Including success--it sounds like you're just an ambitious person, and that's not really type dependent at all.