German and English have a distant common ancestor (spoken roughly 2000 years ago) but neither is the ancestor of the other. There’s no meaningful scientific reason to say that German is the “main branch” of the Germanic languages and English is the “fork”.
Also, it’s not clear that either is “more complex” than the other across the board. German has more complex noun morphology (cases, etc) whereas English has more complex phonology, for example.
I do think English has deviated more from its Germanic roots due to the pervasive influence of French (there are people who call English a creole although I think that's taking it a bit far).
But I agree that languages with more complex morphology aren't somehow "better", that's just weird elitism coming from an era where every language was analysed as if it were some variant of Latin, Greek or Sanskrit.
> I do think English has deviated more from its Germanic roots
That’s fair, but so have German and many other Germanic languages. For example, Proto-Germanic had six cases whereas German only has four (and colloquially spoken German mostly only has three). Dutch has only vestigial remnants of a case system and Afrikaans has none at all.
The Norman invasion of England in 1066 led to extensive introduction of French vocabulary into English. That's a large part of the reason English has so many Latin cognates that you don't see in German (and German also has been purged of some Latin cognates at times as well).