The Shield may have been the greatest faction WWE ever created from the ground up.
Evolution's Ric Flair and HHH were well established prior to the group:s creation (and in Flair's case he was a huge star nationally before even going to WWE).
There was nowhere to go but down, in many ways, for The Shield. They weren't stale, yet were at the apex of their popularity.
Since then, Rollins has gone in and out of being repetitive and being surpassed ringwise by the likes of AJ Styles and Kevin Owens.
Reigns is resented.
Ambrose is taken for granted.
In each case, the company is divided on each talent's worth while the fans would rather sit there and cheer WHAT is cool rather than WHO is cool.
This wouldn't be the case, collectively (as a group) as they were the Musketeers back then. If one member were pushed, then all were pushed by association. Similarly, if one guy were cheered, then the other two would be cheered by association as well.
The Shield was a nearly perfect phenomena. Today, though, each member has holes in their repertoire that can't easily be covered due to the overexposure.