Of empires and democracies

Ned is troubled by the state of the nation, and, no, not just the fact that Republicans and know-nothings are up to their usual tricks. Ned is a respectful student of history, and he has been trying to think of a successful multiethnic "democracy", or more accurately, republic. He can't think of any.
The only successful multiethnic societies he can think of have been empires. Empires are ideally structured to be successful, since the autocratic nature of the government means that it can be equally unpleasant to all ethnic groups, and so be unbiased. The Roman Empire comes to mind, as one of the most successful governments in human history. Gibbons describes the 2nd century AD as the "happiest era of mankind," but then he wrote that in the late 1700s.
Ned thinks of the Ottoman Empire as well, which was relatively tolerant of various religious beliefs and other cultures. This government lasted more than 400 years, and only collapsed as a result of WWI. The same could be said of Austria-Hungary, which managed to keep a bewildering array of contentious ethnic groups at bay until, again, WWI, which was very hard on empires. The British Empire, likewise, was very successful, again owing its demise arguably to WWI.
Ned's point is that he fears for the long-term viability of the US as a multiethnic society, as long as large numbers of new, poorly-educated ethnic groups have to be constantly assimilated. Furthermore, the tendency of politicians to try to please everyone means that some very tough issues, like illegal immigration, education and welfare, not to mention health care for all, may be dealt with unsatisfactorily or not at all.
Those western democracies that have seen the influx of large numbers of alien ethnic groups, France, The Netherlands, and, increasingly, Spain and Italy, are having a devil of a time dealing with the issue.
Ned will watch the unfolding of events with trepidation and great interest as long as he is permitted by fate to do so.