#4. Another superstar will be built to take that spot
At any given point of time, WWE is always looking for the "next big thing". They have the most talented roster in their history but there is a clear lack of organization and a clear lack of creative implementation to build stars bigger.
That's why they say that John Cena is WWE's last megastar. In a way, it is true. While Roman Reigns is undoubtedly the biggest full-time star in the company, he never reached that level of mainstream success that John Cena, The Rock, 'Stone Cold' Steve Austin or even Hulk Hogan did for that matter.
The reason for that is because WWE wants the brand to be bigger than any star. Regardless, they'll be nurturing and building someone up to take that spot once they're ready and when Seth Rollins is beginning to wind down.