I think the issue at the core of this thread is you've picked probably the Shield's lowest possible point since breaking up to declare it all a failure. The core idea here would make less sense six months ago, and I would wager it will make less sense six months from now (potentially even six weeks from now).
Assuming we're talking from a pure creative perspective and not a business one (otherwise this thread goes right out, since the ability to fill up half the cards with Shield guys in different matches and move tons of merch has been bread and butter for business since they broke up), I think you're fundamentally correct that each guy has had some problems, but not to the point that it was a mistake to ever try.
Characterizing Seth as one of the worst WWE Champions "since the Miz" is flawed in a number of different ways - the first of which in my mind is the implication that the Miz was particularly bad as WWE Champion, which I would disagree with, but that's not the point we're discussing. With Seth the WWE was able to accomplish something tremendously difficult in the modern era - getting over a mega heel. Take a look at the annals of the WWE Championship over the last few years or so, and you won't find many first time WWE Champions who were bona fide heels. The only one that really counts as a true heel is Alberto del Rio, and he was never over at the level Seth was. There was a real period of time in between WrestleMania and SummerSlam where Seth got some of the best pure heel heat any wrestler has gotten in a long time. It's hugely difficult for heels in the modern era to straddle between "go away" heat, so to speak, and getting too over with the crowd to the point they're cheered. They booked Seth and his feuds so well through this time period that he really hit the sweet spot for a long time. Now, where you're not wrong entirely, is that at the end of his reign, he was matched up with subpar opponents - Sting, who the crowd never fully invested in, and Kane, who is Kane. But truthfully, I think this situation is only magnified because Seth tore his ACL. Seth was about to embark, presumably, on a tear that would have involved several excellent matches with Reigns and very likely a huge WrestleMania feud with HHH that would have turned him face, finishing off a storyline two years in the making. I think it would have gone down as one of the best booked stories in years for the WWE. Seth's injury cost us that, but we would never have been even close if not for the Shield breaking up.
For Dean, characterizing him as "decently over" does a wild injustice to the single most over full time babyface on the roster. Even on an injury free roster (mostly meaning, stacked next to Cena) Dean is more over with more of the crowd that anyone else is, and is positively approaching Daniel Bryan levels of popularity. For a while, Ambrose losing in big spots looked haphazard, random, and meaningless, and perhaps provided the initial spark to get him this over. Now the WWE has latched onto the idea and are weaving it pretty masterfully in my opinion. Dean's feud and match with HHH was great, and his feud with Lesnar is turning him (not subtly) into Mankind for this generation. I don't think, as has been said here, that Dean needs the win over Lesnar to go over to the next level. Dean isn't a character that's wholly about winning and losing anymore, at least not on pinfalls - Dean's begun to survive on winning the war, not the battle, again in a very Mankind like way. And much like when Mankind finally won the WWE Championship, Dean's win someday will be a huge, business defining moment. Again, totally impossible without the breakup of the Shield.
Reigns is of course the most polarizing member of the Shield and I'm not going to go to bat for him, because I would fundamentally agree that he's been handled badly. I don't think it invalidates the Shield's breakup that he's failed from a creative perspective, but it also doesn't really make a strong point for. What I can say about Reigns is this - he's a very good wrestler who's been horribly miscast as a face, but at the drop of a pin he could become the biggest monster heel potentially in the history of pro wrestling. He's got the look, the moves, and already has the heat - if the WWE would just book it, Reigns would be a massive success as the top heel. He would be the successor to HHH that Rollins never truly could be (hey, does that sound like a good storyline idea to anyone else?).
To wrap it up, the Shield are, in my opinion, a heel turn and a repaired ACL from being the three most over guys in the company and truthfully some of the best booking, analyzed over the long term. As it stands without those things, the body of work (at least for Ambrose and Rollins) and the potential in front of them still is a massive, ringing endorsement for the Shield breakup.
