You're confusing languages "descending" from others and dialects (varieties of a language). It's true that there's not a strict boundary between dialects and languages but thinking English is simply a dialect of Proto-Indo-European is muddy. The better version of the argument is that English is a dialect of Indo-European. Proto-Indo-European would be the variety of Indo-European spoken at a particular time (though it's in fact entirely reconstructed). To follow the linguistic analogy, popular programming languages are creoles (like English). They combine features of multiple languages.
I hadn't thought of English as a creole, even though it combines features of multiple languages, because there's no evidence of a pidgin evolutionary state. But, like a creole, it does have a substantially more systematized grammar than either its Scandinavian substrate or its French superstrate, both of which, for example, inflect for gender. And it certainly shows evidence of rapid historical change, like many creoles, and some of that seems to have been driven by decreolization-like processes. So maybe English really is a creole.
The linguistic analogy starts to break down there, though. In some sense all programming languages are pidgins, and of course they are conlangs.
