Hi Tim
If the number of mutational changes is above a handful given generation times of years there is a limit to how many evolutionary changes can be fixed in populations.
This does not allow for evolutionary searches which the empirical evidence shows can be in the thousands or more changes to form a new gene. Until a new gene is formed that can help with reproductive advantage then the waiting time to fixation can be beyond the edge.
There is most likely an edge but where that edge is, is still uncertain. Having two sides to this argument appears to be valuable.