I guess the point is that it was very very unlikely to find someone with the needed skill set by chance, especially since Maz made it sound like the Master Code Breaker was the only one who could do it.
Maz said that he was the only one she could trust to do it, not the only one that could do it. It was established that Maz herself could have done it, but she was unable, at the beginning of the transmission.
Good point. I guess it’s hard to say exactly how rare the code breaking skill is, based on what we see in the movie. In fact we see three very different characters who could have done it, so maybe it’s more common than one might think.