While Farah's 3:28 (and recent medals etc) certainly show he is the best runner in the world, I would still bet on the field over him.
He's lost a 5000m already this year, and narrowly won another. I don't think he is dominant enough to just completely run away from everyone. All it will take is just a little bad luck for him to lose as the margins are fairly slim. E.g. in London's 5000m, at the bell Gebremeskel was mid-pack and boxed in, he eventually had to maneuver out to the 2nd on the bend but Farah had a 15m gap already by that point. If Geb had a better position or Farah got boxed at the bell, and Geb runs away with it. I think if 6+ Kenyans/Ethiopians/Qatari are bunched with him near the bell, a sprint finish has pretty good odds of one of them winning. With just slightly better luck/positioning Alamirew can outkick Farah.
How I see Farah getting beaten:
10000m: Complete kickers race. Kenya/Ethiopia give their star kicker the pole position, the other two protect them from getting boxed (e.g. making Farah et al run wide so that they have time to respond to any moves and make him run the extra distance). I'd say Gebremeskel has a good shot of beating Mo, and maybe Jeilan will turn things around.
5000m: ideal strategy, turn it into a 4000m team time trial. if Farah settles into the back like he did at the Olympics [ if Ibrihimov had the fitness he could have run away with things as he had a ~20m lead on the 2nd/3rd lap and could have easily stretched that out...or at least run away from Farah]. I think Ethiopia and Kenya will have someone within 2-4 seconds of Farah's fitness for a 3-4k that could succeed with this strategy, especially if another team member will make the break with them... plus, Mo will have the 10k in his legs. Otherwise, I'd still take the field over Farah in a kickers race.