I never really understood at what point a language evolves enough to be an entirely new language.

Old English feels so far removed from even middle English, let alone modern English.

We have "new" and "old" to differentiate them, but with how many Latin words alone entered English between Old English and Modern English, It's something I've never found a comprehensive answer to.

I guess, what is it about proto-indo European that we acknowledge as a distinct language from the hundreds of thousands of languages that evolved from it, other than time scale and global impact.