I'll try to be brief because I don't want to derail the thread.
@mudcrabmerchant: It looks like you agree with me that linguistic differences are often a very good way to draw cultural boundaries, and that cultural identity depends on language to a considerable extent. But no-one claims that there must also be a “perfect match up” so I am not sure what you are complaining about. Regarding what you said about the Ugro-Finnic stuff, I don't dispute any of it, and don't see how it affects my point.
@classicist:
(1) I agree that criteria based on ethnicity are often troublesome.
(2) I don’t think the Ugro-Finnic example disproves anything. The two languages have diverged too much, and the similarity between them just isn’t good enough when it comes to comparing contemporary cultures (I believe that the similarity is so subtle that it takes an expertise that goes beyond the competence of most native speakers to see it—being a Finn yourself, perhaps you can confirm this). So it isn’t really a good counter-example. Clearly, assessment of linguistic similarities has to take into account the evolution of languages over time (which the OP did).
(3) Regarding your Finnish/Swedish example: no-one claims that language must be the one and only criterion. Obviously there can also be other criteria such as geographical proximity. However, I would say that linguistic similarity usually takes priority over other considerations. For example, all else being equal, who would you say is more culturally similar to you, a neighbour who speaks your language or one who doesn't? And even if my priority claim is wrong, the relevance of linguistic similarity still stands.
BOTTOM LINE:
There obviously is a strong correlation between culture and language. Good linguistic evidence (such as that provided by the OP) is nearly always relevant when arguing about cultural identity. The usual complaint, "but culture is not language", is a straw-man.
@mudcrabmerchant: It looks like you agree with me that linguistic differences are often a very good way to draw cultural boundaries, and that cultural identity depends on language to a considerable extent. But no-one claims that there must also be a “perfect match up” so I am not sure what you are complaining about. Regarding what you said about the Ugro-Finnic stuff, I don't dispute any of it, and don't see how it affects my point.
@classicist:
(1) I agree that criteria based on ethnicity are often troublesome.
(2) I don’t think the Ugro-Finnic example disproves anything. The two languages have diverged too much, and the similarity between them just isn’t good enough when it comes to comparing contemporary cultures (I believe that the similarity is so subtle that it takes an expertise that goes beyond the competence of most native speakers to see it—being a Finn yourself, perhaps you can confirm this). So it isn’t really a good counter-example. Clearly, assessment of linguistic similarities has to take into account the evolution of languages over time (which the OP did).
(3) Regarding your Finnish/Swedish example: no-one claims that language must be the one and only criterion. Obviously there can also be other criteria such as geographical proximity. However, I would say that linguistic similarity usually takes priority over other considerations. For example, all else being equal, who would you say is more culturally similar to you, a neighbour who speaks your language or one who doesn't? And even if my priority claim is wrong, the relevance of linguistic similarity still stands.
BOTTOM LINE:
There obviously is a strong correlation between culture and language. Good linguistic evidence (such as that provided by the OP) is nearly always relevant when arguing about cultural identity. The usual complaint, "but culture is not language", is a straw-man.