I agree Wiggo should have been in the TdF team as a superdomestique but I'm not convinced he'd have been any better as a Plan B than Porte.
I'm also not convinced that having a Plan B is such a good idea anyway. It's fine if you have, say, Hinault and Lemond in your team but on the other hand, not if you have, say, Cadel
Evans and Tejay Vangarderen, as BMC did in last year's Tour, to the benefit of neither.
Being able to implement Plan B, should the need arise, is another matter.
Geraint Thomas might have had a chance of winning the TdF in 2011 if Sky hadn't made him wait when Wiggo fell off. Froome definitely would have won the 2011 Vuelta if they hadn't made him wait for Wiggo on the Angliru. Although the way he almost overturned the deficit on Peña Cabarga shows what a classy racer Froome is when he's allowed/needs to follow his instinct rather than a strict by-the-numbers gameplan.
To be fair to Brailsford, I don't think he has ever claimed to know much about road racing.