First off, stating that Seth Rollins has no charisma I feel is simply unfair. Whether anyone loves or hates the guy, I think very few can deny his in ring talent, his skills or the work ethic. Furthermore, considering he was part of arguably one of the most succesful factions of all time, aka the Shield, basically proves the opposite cause the Shield don't become who they become without certain charisma and that obviously goes for Roman Reigns and Dean Ambrose as well. Is Seth Rollins the best promo? Certainly not but, again, saying he has no charisma is a bit harsh...
Secondly, RAW ratings have been in the shitter for months now and in a constant downfall for years and that ain't on any Seth Rollins or any other superstar in particular for that matter. If you need to find someone to blame, take it on the creative "geniusses" (including Vince obviously) who lack exactly that, creativity. And even when they get something good started, something intriguing, they still somehow manage to fuck it up in the end nine out of ten times...Take Ambrose for example and what followed his initial epic heel turn...Take AOP and the...piss jokes they became alongside the entire tag team division...take all the talent they don't use at all for the simple reason they have nothing for them...take the talent they DO use and how they either mistreat or simply fuck over, like Rousev or Neville earlier who simply walked away...The (shit)list is endless...
You ask for a new Batista or a Goldberg or a Cena? I'll add a Rock or a Stone Cold in the conversation but the fact of the matter is these type of larger than life characters are now no more because WWE have miserably failed to create new ones. And even when they did try by forcing the situatiuon with Roman Reigns, well, we all know how much of a success story that was...
Bottom line is this: Even if Seth Rollins is not the ideal choice to face and beat Brock at Mania (and I am not saying he is), really, who else is there on RAW anyway? I could only possibly argue a face Drew Mcntyre which is an idea I'm not sure WWE are keen on at the moment or maybe Demon Balor. Problem is, we haven't seen Demon Balor in ages and, well, Vince doesn't believe...