Or is the process of disappearing/extinction of ethnic groups too varied to make generalizations about?

The only question here that is focused enough to answer I think. Yes. Another difficulty is trying to adequately define what 'ethnic group' actually means. If an ethnic group is subsumed linguistically into another culture (as happened for example to the Sumerians) does that ethnic group still exist if they retain other cultural and genetic traits? Does an Assyrian today have any relationship to an Assyrian of 1000BC? L

Let's assume that we agree that an ethnic group retains some measure of cultural (including linguistic) identity and a measure of genetic similarity. In that case, we can make the general statement that it is a case of a stronger culture/ethnic group absorbing a weaker one. In this case, stronger can generally be defined as superior martial and/or economic strength. For example, Rome profoundly changed the face of the Italian peninsula, leading to the virtual extinction of separate 'ethnic' groups. They didn't sell them all into slavery, so this wasn't a violent extinction for the Romans. A good example to consider is the Cimbri, who were wiped out by the Romans. Even if some survived, they would have been subsumed into other tribes. Are they a dead ethnic group? Were they ever a distinct ethnic group or merely a small part of a larger one? Or, we can use the modern 'cimbri' of norther Italy, of which there are only 2000 speakers left. Here's an example of economics taking their toll. If they stop speaking the language are they still an ethnic group?

Sorry, we don't allow thoughout history questions.