The grouping isn't based on grammatical similarities exactly. (Generally the grammatical properties are similar though!)

Genetic classification is based on history only, where the language originated. A language might in theory lose all of its vocabulary or have extreme grammatical changes due to contact, and it would still have its origin in the same "genetic" ancestor.

For pidgins and creoles, the situation is a little more complicated (and those really are the most extreme examples of borrowing and changes). For this reason, many linguists don't try to put these languages into genetically based groups. (Instead, they use geographic or other classifications, like what the lexifier is.)

There are really two separate questions here:
1. What is a genetic grouping in Linguistics? [answered above]
2. Why would anyone want to do that? What would they use it for?


I think this can be explained by going back to the history of historical linguistics.

Sir William Jones suggested a number of languages were related including Latin, Greek and Sanskrit, due to what appeared to be systematic similarities:
https://en.wikipedia.org/wiki/William_Jones_(philologist)

Following that, others, including Jacob Grimm, discovered various systematic correspondences in these sub-groups. For example:
https://en.wikipedia.org/wiki/Grimm%27s_law

These systematic correspondences indicate (systeamtic) sound changes that occurred in the history of the languages. So Germanic F/TH/H correspond to other Indo-European languages with P/T/K. By going a few more steps in the reconstruction, it is believed that originally a language we refer to as "Proto-Indo-European" had sounds P/T/K (still surviving in, for example, Latin and the Romance languages, among various others) and that at some point Proto-Germanic had a sound change. So in all Germanic languages (English, German, Icelandic, etc.) we have words like "father", while in the other groups they have words like "pater".

By looking at genetic groups in this way, systematic correspondences can be identified and the history of languages, and some grammatical properties of their ancestors, can be deduced.

If we didn't use genetic groups like that, none of those methods would work. If we borrow a word from French or Latin into English, it wouldn't have the same changes! Consider "paternal" (a borrowing) as opposed to "father".

So the answer is that genetic classification is useful for reconstruction, among other motivations.


Another less interesting answer might be that in general lexical items (words, vocabulary) are not given very much weight in terms of grammatical theory or otherwise. They're just the details, and they're arbitrary. There isn't a legitimate reason to say this is the "right" way to do it, but it's just one way to classify things.



There are other times when we do consider borrowing and so forth. For example, there are very limited written records of Basque before relatively recent times. But by looking at words borrowed into Basque from Latin (or early Romance dialects), we can actually figure out certain properties about the history of Basque itself. We know what the Latin borrowing was originally, and we know what it is now in Basque. So if we see a difference, and that is probably not due to the way it was borrowed originally (for example, if a sound in Latin also is in Basque, but the modern Basque borrowed word has a different sound in it), then we can date some sound change to after the time of borrowing from Latin. These cases are relatively rare and more complicated to deal with, but the point is that different classifications, borrowing, and so forth can be relevant. But they don't replace genetic classification for the purposes that is usually used for.

It's confusing to change a post after someone has already replied. You can reply again, but editing the earlier post makes the conversation make less sense to others who might read it later.

But your rephrasing seems to be essentially the same as the original question, so my answer should still apply.

It's comparing apples and oranges. Describing the percentage of borrowed vocabulary in a language is relatively easy, but trying to figure out what percentage of a language is vocabulary (or borrowed vocabulary) isn't a coherent question. Is English 10% borrowed vocabulary? Or is it 60% ignoring grammar entirely. There's no way to pick a formula for this.

But yea, of course, you can say that English has Romance vocabulary and influence. Or even "it's a little but Romance" but it is unclear to me what that really means aside from being essentially a historical footnote.

No one would suggest ignoring the Romance influence when it is relevant. But it just doesn't make it suddenly "become" Romance, whatever that would mean.

The other two answers:
1. That's just not how people traditionally approach this question. You can do whatever you want, but that's what is normally done.
2. Vocabulary (lexicon), rather than grammar is often not considered to be as important as the other, grammatical aspects of a language. One reason is that vocabulary is arbitrary while grammar is often thought of as having rules, patterns, or even universal properties across languages.

You're welcome to disagree with any of this but it is mostly pointless to argue about defining an established concept differently. Genetic classification and language families are standard. If you want to do it differently just talk about something else like percentage of borrowed vocabulary. Nothing wrong with either approach. They're just different. But picking one over the other is pointless because they're not the same thing or answering the same question.


A better solution to all of this might be to establish a multi-part typology of language relationships including speaker populations, geographic distribution, typological features, etc., in addition to grammatical/genetic relationships and vocabulary.


On a more technical level, this might answer your question: historical linguistics is interested in inherited patterns that can be used to reconstruct earlier forms of the language and show systematic relationships. Because of the messy nature of borrowing, of any kind whether grammatical or lexical, it is a hindrance to that goal and historical linguists work hard to avoid it. And vocabulary is the most easily borrowed part of a language so it is rightly excluded for that reason. Syntactic and morphological borrowing also can occur, as well as rarely borrowed sounds. These would also be excluded, but they just aren't as pervasive as vocabulary.