Fair point. However, I think even if we assume that he will be drawn to play both of them in only two majors, the odds will be against him capturing another major.
The stars aligned for Federer this year at Wimbledon. Nadal taken out early, a slightly undercooked Djokovic in the semi-final, and a nervous Murray in the final, not to mention the roof coming on at times to not exactly hinder his play, if you see what I mean...
My gut feeling is that another season into the tour, he will need the fates to be extremely kind once more to capture an 18th major.
And Murray, after Olympic gold and his first US Open title, will probably play more freely if he meets Federer again in a major.
I would be surprised if Fed won a slam next year, but that story about "stars aligning" can be said for many slam winners.
What if Fed had not defeated Djokovic in RG 2011 final ? would Nadal have won ? the semifinal depended on a very few points, and Djokovic was in a great streak against Nadal, and Nadal felt unconfident against him, as it showed in Wimbledon final.
What if Fed had converted his match points against Djokovic in the USO 2011 semifinal ? Moya and Toni Nadal firmly believe that Nadal was the real loser that day, not Fed.
Djokovic's win in the AO 2012 also depended on a few points
etc etc ... most slam wins partly depend on luck, very very seldom it's not the case.
There's a big contradiction between the big part of hazard in all that and the fans' definitive assertions about "this will never happen" etc ...
And Fed could be luckier than having to face Djokovic and Murray to win Wimbledon. For instance he was luckier in AO 2010.
I don't get that idea that some have that Fed's success or not only depends on Nadal : outside of clay, he can very well defeat Nadal anyway, and he can very well be defeated by many other players : Djokovic is by far the favorite against him, Murray has a leading H2H, Berdych, Del Po, Tsonga ...