Mainly in terms of the sheer number of loanwords, I think. But the important thing to remember is that no natural language, living or dead (or any state in-between), is pure to any reasonable standard. (And constructed languages, both those intended for human use (auxlangs and artlangs) and computer languages, are usually mixes as well.)
“Old English” is a common usage and yes, it is specious.
But it’s unlcear to me that the statement “English is no less pure than… Spanish…” is correct. I haven’t done a word count, but I would suspect English has a higher percentage of words known to have come from another language than Spanish does.
Why?
You’re quite right, although I’ll add that English has a greater total number of “original” words than any other language. Remember, the 19th and 20th centuries have given the world a broader swatch of new ideas and inventions than any other period in human history, and all this new stuff had to be named. Most of that naming was done in English, and oftentimes the English name became the pejorative.
I argued that English is a cobbled-together language in the sense that it borrowed (and continues to borrow) from more languages than any other; Spanish is mostly a fusion of Latin roots and bits and pieces of the languages of the tribespeople who lived in and around Spain when the Romans showed up, for example.
The thing with English, though, is that all that borrowing and creating has given it much more depth than other languages; you can speak “pure” English if you want, relying exclusively on Latin roots, or speak “pure” English relying solely on Anglo-Saxon roots (though the second is much more difficult).