If our strategy for reaching the peoples of the world moves too fast, goes for shallow-instead-of-depth, and cannot scale to reach the individual people within those people groups, the cost of the strategy is too high.
I believe "reaching the peoples" should be shorthand for "inculturating the Gospel into a people group so that it can grow, spread, and reach all the people within the group."
The way I've sometimes heard it described, the "end line" of "reaching the peoples" is the the "inculturation" (e.g. "moving into the neighborhood", to use the John 1 Message version). I believe "missiological breakthrough" is a critical step, but not the last one. The end line isn't reached until all the individuals have the chance to be made disciples, and the responsive ones are being discipled. This is the command outlined in Matthew 28.