I honestly couldn't say how large is the actual concern about what happens with the Wrestlemania main event for the World Heavyweight Championship, but I do have to wonder about it. I honestly can't speak as to whether or not Roman Reigns is indeed "ready" to become WHC, but it's undeniable that in Reigns vs. Lesnar, you have someone who is still basically a rookie with a limited singles background fighting a man who has demonstrated a lack of interest in an active wrestling career & who there are still rumors will be leaving the company. Either way, the company is taking a big risk.
Of course, there is another option: Lesnar defeats Reigns or loses by DQ, keeping the belt. Suddenly, Seth Rollins clocks him with the MITB briefcase, uses the title shot, pins the unconscious Lesnar and becomes the new WHC. Sure, Rollins is a hated heel, but people might feel that he had the seasoning that Reigns still lacks, without the time lost due to injury. Reigns can still chase Rollins for the title throughout the Spring and Summer, while Lesnar can go and do whatever he wants outside of the WWE.
Of course, there is another option: Lesnar defeats Reigns or loses by DQ, keeping the belt. Suddenly, Seth Rollins clocks him with the MITB briefcase, uses the title shot, pins the unconscious Lesnar and becomes the new WHC. Sure, Rollins is a hated heel, but people might feel that he had the seasoning that Reigns still lacks, without the time lost due to injury. Reigns can still chase Rollins for the title throughout the Spring and Summer, while Lesnar can go and do whatever he wants outside of the WWE.