If Nadal fails to win another Slam, Federer will have won the contest - hands freaking down!!!!! Lead of four almost guarantees it, Nadal's winning H2H and other advantages notwithstanding. And that's without No.35, surgery, six-month hiatus etc. Add that and it's as close to without reproach as it can get.
However, if Nadal adds No. 15 with No. 10, Federer's free reign may come under question - again. Lead of 'mere' three - with No.10 and undisputed No. 2 slot on the Slam pole - may seem even slimmer than it was when Nadal 'was' @ No. 14.