He didn't invent the concept. It was a common idea in his lifetime. He just used it a lot.
And there is, i've heard, no evidence that he ever said or wrote anything that could be translated anywhere near literaly to "entities should not be multiplied beyond neccessity".
A good selection tool for theories. Not an indicator of truth. Easy to over-apply.
For example, the theory that there are a bunch of people out there reading this message and possibly replying to it is far more complex - particularly in the multiple-entity sense, but also in others - than some other explanations of what is happening here. A decent sim, for example. Or just one guy - I'm thinking maybe Ashton, with his very distinctive use of language as a pretty good cover for all the more formal-writing side-personae, except that what is identity when you are the whole group?
No, I'm rejecting the razor - as William himself would - in favor of the more complex hypothesis here.