I think the more fundamental question than "can code be secured" is something along the lines of "can we properly measure the security of the various components of a sufficiently codebase". Security consultants have workflows but ultimately everything is ad-hoc and suffers from cominatorial explosion, and what's missing is the security equivalent of Lean, a real formalized effort to prove security. There have been many attempts, of course. One of the great problems of our era.