I'm not disagreeing with you. (In fact, I'm happy with the position that Ambrose has found himself in.) I don't even disagree with the idea that WWE should look to someone like Reigns as an ambassador for the brand.
My complaint is how external marketability manifests itself within WWE programming. Roman Reigns or John Cena can be the face of WWE to the outside world without constantly sniffing around the heavy-weight title.
In other words, its perfectly fine to have one guy represent WWE, AND hold a major title... so long as he's not the only face capable of it. Creating a singular hero with a constant path to the championship creates stagnancy, predictability, and dependency.
These problems may not effect investors or advertisers, but that's not to say they aren't problems. Putting the vast majority of your eggs in one basket can easily backfire if God-forbid, Roman is injured or moves on to movies. It can lower backstage morale. It can alienate fans of popular, if underused talents. All of this can add up to a variety of problems from shrinking ratings, merch sales, and unwanted reactions at live events.
Moreover it creates a nightmare for creative to write a compelling narrative. A powerful face needs powerful rivals. So, in order to create powerful heels for ppv showdowns, the bad guys need to decimate other faces on their way to the top. That's why you see guys like Ambrose and Ziggler become props for Reigns feud. That might not be a big problem, but if there's only once face at the top, he'll need fresh rivalries... and once a heel is defeated his threat becomes diminished then a new heel will be needed to replace that threat. Repeated too often and the other faces on the roster look pathetic and will never be able to regain their credibility (relegated to feuding in an endless cycle of meaningless rivalries against the main face's throw-offs). The only way I can think of to avoid this problem is to create nearer-parity among a small field of top-level competitors who each have a believable chance at winning the WHC.