#2 Fail: No proper factions to face, will become stale

One major issue with the Shield reuniting is the lack of proper competition for them. Say the Shield does reunite, who do they go against?
The only legitimate faction that they can go against is the budding heel faction of Drew McIntyre, Baron Corbin, and Bobby Lashley. Though it would be great to see The Hounds serve justice and beat down these heels, they are in a feud with Braun Strowman.
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Even if The Shield does collide with them, what next? There is no one whom the Shield can go against.
#2 Success: Create Buzz for WrestleMania season

If there's one major reason why Vince might plan to get the Shield back together, it would be to get the mainstream attention on WWE's product and create the Buzz for the WrestleMania Season.
The Shield will no doubt be one of the biggest attractions of WWE shows and will surely attract more fans to come and watch these shows.
With the recent loss in TV ratings as well as low attendance in RAW and SmackDown, The Shield might just be what WWE needs.