“…languages can be imposed by a small minority…”
Irish (i.e., Gaelic) does not have a simplified grammar such English or Spanish which is what we would expect for an imposed language. It’s a language that is learned at one’s mother’s knee.
I think there is a general tendency among the Indo-European languages (or at least found in many of them) to reduce the complexity of inflections over time. All of the Romance languages have simplified the inflections inherited from Latin. Modern Greek still has inflections but not as many as Ancient Greek. English has very few inflections (Anglo-Saxon had many more)--German has simplified its system somewhat but is still more complicated than English. But I doubt that the simplification of English has much to do with the language being imposed on others, although during the period after 1066 when the upper classes spoke French, that may have had some influence on reducing English inflections.
Afrikaans is derived from Dutch but has simplified grammar, or so I understand. Supposedly that was the result of children being cared for by native servants who did not know Dutch well, so that Afrikaans sounds like baby talk to someone from the Netherlands. Or so I have read.