This goes back to how every WWE faction in history works... they have 3 levels...
The Must Get Over:- The purpose of the faction, he may not be the "front man" but he he is the one who, when all is said and done has to be elevated from it. When it was the Nation, it was the Rock, DX post Shawn injury it was Hunter, Evolution it was Orton, Legacy it was DiBiase... even the Spirit Squad with Nicky and Kenny... one is the current IC champ in case you forgot... In the Shield this was Ambrose... from day one he was given the role of "leader", he was the one given the singles title and the one everyone thought from NXT all the way to the night Rollins turned would "be main event first". Indeed Ambrose is, defended or not the 4th longest US champ in history... that is a serious step, not given to a wannabe or a gamble... that is something given to someone you want to be "the guy".
The Guy they WANT Over:- Go back to Evolution... This was Batista... to Legacy it was Orton in the Wyatts it was until recently Luke Harper and old DX it was Chyna... Nation was Faarooq.
This is the person who they want to get to that point, but it's not as important as the main guy... in the Shield that was clearly Reigns, all the old "look" reasons come into play and also his lineage to the Rock and his family... of course WWE would want him to get over, but he was never the main plan but "plan B", which is ironic considering that terms use in Rollin's push. Reigns was given the Surivor Series last man standing moment and the Royal Rumble performance... these don't quite add up to the US title reign Ambrose had, but they are commonly used tools for the guy they WANT to get over... they have done it before... Think back to the Nation with Farooq injuring Ahmed... he was given some big moments even though he never won as often as The Rock... and once he was out of the Nation was given the APA... which had a LOT of moments.
Then finally...
The Guy Who IF they get over - they will...
These are normally the bonus guys in the factions... The Outlaws and X-Pac in DX, Erick Rowan in the Wyatts, D.Lo in the Nation and Cody in Legacy.
Make no mistake these guys very often DO get over, sometimes more than the person the stable was intended to help. No one predicted Cody would not only be still around but one of their most reliable and versatile hands today when Legacy was the focal point, it was geared towards DiBiase, but Cody was patient... just as RoadDog or D.Lo were. At one point in 1999 D. Lo was not only the EuroContinental dual champion but quickly getting as over as some of the other top guys. It was only the accident with Droz that killed that momentum. D.Lo was on his way, if Henry got a title, D. Lo without that accident would have one day...
Rollins is clearly that guy in the Shield. He was the guy who was "most unlike" WWE's type on signing, he had the ROH title/pedigree so was perhaps seen as a Punk clone rather than something different like the other 2. It's ironic that Punk's walkout has seen them push Rollins as his direct replacement. The gave him the tag belts, the catalyst role for the split AND the MITB Briefcase... but you still get the feeling he won't actually win the title ahead of Ambrose or Reigns.
The Shield was perhaps the first time they have legit tried to push 3 guys to the same level...even Nexus had a definite order...but be sure that was not the plan when they put them together, it perhaps only began when WWE realised that ALL 3 could not only get over but potentially replace their 3 Attitude Era icons...
Ambrose is the new Austin without doubt, he's already getting the "undercurrent" of support Austin got, if they let him go a little bit more edgy and resist the temptation to let Austin come back and stun him... then they have their new guy.
Reigns has the potential to be the new Rock without doubt and Rollins is clearly being moulded into the new Triple H style heel.
Of the 3 Ambrose is the furthest ahead, near ready now and more to the point, over naturally rather than the force fed way the other 2 are... The fans had a choice 18 years ago between Shawn, Austin and The Rock...the other two were forced on them and Austin wasn't they chose Austin... and they will choose Ambrose in the same way... the other two still got over in the end... but WWE survived cos the fans chose right... They're in that kind of spot again.
Ambrose is quickly becoming "watercooler" level, where you talk about what he did... Reigns isn't... Rollins isn't...Rusev isn't... so that means Ambrose is safe.