That's not necessarily true. I can't comment on Ruthenia or on other places, but at least in Lithuania, Polish-speaking nobility retained a distinct "Lithuanian" identity well into the 18th and 19th century, and even, in some situations, as far as the 20th, well into the Lithuanian National Revival (Stanislaw Narusewicz, Michal Romer and other krajowci are a good example of this in the modern era). Of course, there was the overarching "Polish-Lithuanian" identity above it, and by the 19th century, it had become synonymous with Polish in Western terminology, but simply stating that a person who adopts the Polish language immediately becomes a Pole isn't necessarily correct.Contrary to the UK, here there be a trend that anyone who adopts the polish language also identifies himself as a Pole. And adding catholicism to that, it means even more "polish". The British Islands have their own specifics, the Eastern European Plain has its own.
I personally stand by the idea that Lithuania would be Poland's Scotland. Speaking the same language, member of the same federation and more or less supporting that federal identity, but still fiercely standing behind their own identity, be it in traditions, accent or reviving their old language and culture.