I see what you're saying but I think there are a few things to be cautious about. First, only a certain percentage of EDs across the country are high acuity and high volume and can properly leverage EM boarded docs. Logic would have it that as the number of EM boarded docs has increased over the last couple decades, they have started filling out EDs from the top (ie busy and high acuity) down. Only by working at a busy place can an EM doc make 300k+. If the end goal is to staff every ED across the country by EM boarded docs, then those docs are going to have to accept much lower salaries, because sleepy rural and suburban emergency departments aren't going to throw out their 200k fm docs and replace them with 350k EM docs.
Secondly, as you noted, EM is a very new specialty, and significant numbers of grads haven't entered the job market except for very recently. This means there are few physicians leaving the field each year, so every new graduating class of residents is almost entirely a net addition to the labor pool. The job market is holding steady so far, but if it ever does overshoot, it's going to overshoot big league and get worse and worse each year for a very long time. I think caution is warranted.