Was austin different? Hogan? Sammartino? Backlund? Flair? Goldberg? Inoki? Tsurta? Misawa? Lawler? El Santo? Mistico?
All machines that companies built up guys to feed to, in their time, it was their world and everyone else was living in it.
While this is undoubtedly true, the modern era is different for the simple fact that this is the first generation of fans who have been raised (for lack of a better term) on the concept of change as the only constant. When a Backlund or Hogan had a long title reign, it was what was expected. Champions reigned for a long time. The same with the regional champions like Lawler or Dusty. Fans wanted and accepted that status quo.
The modern fan is different. The last 15 years have been centered around the concepts of change and surprise. Title change hands with more regularity, alliances change and shift. The end result of things continually returning to a single default set up is contrary to this thinking. With this being the case, having one invincible centerpiece who essentially represents a status quo or default status, is not a good idea.
SCSA does illustrate your point to an extent. SCSA was as protected as Cena, maybe moreso. No one ever got over on him or fooled him or out thought him. He rarely lost cleanly and if he did he would get some manner of moral victory or embarrass the winner in some way. But one thing that SCSA was not, was a dominant champion in the sense that he had the belt constantly. HHH held the belt more often and for longer times than SCSA did.
The major difference is that SCSA did not represent the status quo in the way that Cena does. SCSA always won, but his wins were personal and they didn't carry such a sense of inevitability as Cena does. What I mean by that is that with real nearly equal options like Rock or HHH to hold the belt, SCSA being kept strong never felt like he was inevitably going to be champion every time he feuded with the champion or challenged for it. Cena has that sense, in the same way that HHH had it during his last big run, where it feels like no champion has a chance to hold him off.