If you want to establish the origins of a language, there are two main things which have to be taken into consideration: basic vocabulary, and grammar. The former is what would weigh the most, were one to engage in evaluating the average occurrence of each word in the literary corpus of a given language, as you suggested in the question. In the case of English, this is overwhelmingly Germanic. The fact that in the total vocabulary of a certain language at a given time in history there may indeed be more words of foreign origin should not surprise you. For instance, were someone to have created a similar pie-chart for the Romanian language spoken (more than) one or two centuries ago, the majority of its total number of words would have been of Slavic origin, despite the fact that Romanian is a Romance language, descending from Latin, which can be quite easily noticed by evaluating its basic vocabulary, and grammar.