I'm substantially less convinced. While carbon is the main cause of complexity, that's still carbon with other elements. Your options in this hypothetical are hydrogen, helium, lithium, beryllium, boron and carbon and that's it. Helium is effectively out (I think, I don't know enough to be that confident that basic bonding behavior will be that similar when you've drastically altered the fine structure constant.) The chemistry for that set isn't nearly as complicated as that involving full CHNOPS. And the relevant question isn't "can life form with these elements" but rather "how likely is it?" and "how likely is complex life to form"?