#4 We get to see Brock Lesnar more often
Building upon my last point, no matter how much people will criticise the fact that a part-timer in Brock Lesnar is holding on to the top championship on WWE Monday Night Raw, there is no doubt that those very same people still want to see The Beast Incarnate as often as possible.
And, that's exactly what's going to happen if he holds on to the championship till Wrestlemania 34. He will appear more often with Survivor Series, the Royal Rumble, and Wrestlemania all happening within a couple of months of each other and a smattering of important pay per views in the middle of each one; Lesnar will almost certainly have to commit to more WWE dates.
And, at the end of the day, everyone wants to see more Brock Lesnar. This way, we get to see him come in and wreak some havoc on a more regular basis.