Without commenting specifically on the STIR/SHAKEN standards, I can tell you that this issue was observed when EMV smart card technology was introduced in Canada. In the months following the implementation of the technology and once the critical mass had been reached, we saw fraudsters moving south of the border quickly enough to continue cloning cards, whereas here the trend was fading.
For fraudsters, the border isn't an issue. They'll go where they encounter the least resistance and the fewest obstacles to commit their crimes. We can reasonably believe that, if the STIR/SHAKEN standards are implemented in the United States and not in Canada, the fraudsters will come here and spoof telephone numbers.