A problem with your math is the implicate assumption that miles are equal. Setting aside the fact that holders of DL miles, in particular, are increasingly learning that their points don't equate with AA and others, the applicable matter in this case is the obnoxious YQ charges that you'd need to endure in order to make use of your companion certificate (the limitation to BA metal constitutes another strike against BA miles when used in this fashion).

To be fair, in one of the other threads in which this topic has been discussed, someone came forward with a verifiable example of a long haul BA C ticket that required a mere $300 in taxes+YQ RT (still absurd by my standards, though not extortionate), but in most cases that I've analyzed the taxes on C tickets are typically in the same ballpark as economy tickets on the same routes.

In short, while the companion certificate might pencil in some cases, you've made several bold assumptions during your pass through the numbers in your OP.