It seems to me that to put the kingdom of God entirely in the hearts of believers is to minimize the ruling Jesus. Our own lives are a consistent testimony to the inconsistency of Christ’s kingship in our lives. The cynical side of us should say, “If’s that all the better he can do, that’s not much of a reign.”
Yet the biblical description of the kingdom is far greater than that. It is a time when all nations submit to the king all the time on this earth and flock to him for teaching and guidance (Isaiah 2:2-4), not just some people some of the time, and not in some eternal state. It is hard to see how any reasonable reading of Isaiah 2:2-4 fits into a “kingdom now” motif.
The rule of Jesus has to be bigger than simply ruling in the believer’s heart because Jesus is a bigger king than that.
I believe Jesus ought to rule supremely in believer’s hearts. I just don’t see that as being the entirety of what the Bible describes as the kingdom. Rogue nations, open display of depravity, persecuted believers, unpunished sin, and short lives are not what Isaiah describes as the rule of the Righteous Branch. Therefore, it should not be what we teach as the rule of the Righteous Branch.