#1 Time Crunch
Believe it or not, the biggest indicator that The Shield will break up is the timing of it all. For example, rumours indicate that The Shield will be breaking up around The Survivor Series pay per view, which will take place November 18th. With that in mind and the seeds that WWE has already planted with the storyline, it seems like this will be the case.
Another thing that shows The Shield will break up around this time is the fact that WWE apparently wants to do Roman Reigns versus The Rock at WrestleMania 35, which is going to take some time to build. Unfortunately for WWE, that means that there isn't much time between The supposed Shield break up in November and the build to Mania.
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In the end, this most likely means that The Shield reunion has to happen sometime around November, which will thus leave WWE enough time to script the breakup, have the fall out and give Reigns enough time to build for his feud with The Rock. It also gives Dean Ambrose and Seth Rollins to find a role for WrestleMania 35, which is also important.
If nothing else, WWE can't realistically keep teasing The Shield break up for too much longer and still have time to build up Roman Reigns versus The Rock at WrestleMania 35. That means the break up needs to happen sooner rather than later or The WWE will risk overlap in the storylines that could cause confusion.