The burn out rate for EM will continue to drop as it has in the past. The big reason for the burn out rate in the 80s and 90s was that no one was residency trained. People did EM because they liked it, but they were not trained for it formally.
Now with the blossoming of many EM residencies, people go into EM because they like it, but know what to expect from the beginning and are trained for every aspect of it. You get people staying in the EM field much much longer now...
With the boom of EM residencies, and with hospitals/insurance companies requiring board certification in EM, slowly the EDs will be filled with only residency trained EM physicians. But like the above poster said, the real small rural EDs may not attracted the EDs (although I think eventually they all will be filled by EM trained docs).
Right now the outlook is good though for FP trained attendings to work in low volume community EDs. How long this will last is anyone's guess. Mine is 10-15 yrs.