Why on earth do Mclaren have a far bigger chance, the Ferrari is better than the Mclaren this year, with a worse engine, the Ferrari engine will improve, the Honda MIGHT improve over the Merc, it MIGHT be worse, their aero isn't likely to be drastically better than Ferrari's all of a sudden.
Ferrari have a far bigger chance of beating Mclaren than the other way around, there is a exceptional slim chance that Honda make a brilliant and reliable engine in their first year and even then Mclaren will have to leapfrog most teams in aero to be competitive.
You also ignored Williams, with a great engine, decent aero and improvements, without major regulation changes there is little reason for them to drop off significantly. Their upgrades are working, the team has an idea what it's doing, they'll have a better idea of getting more out of the engine next year. Which cars without some significant change(change in engine, drastic reduction in funding, losing a top driver and only getting awful ones) have been great in the first year of new regulations but terrible in the second?
Williams might move back relative to say RBR if the Renault improves significantly and the Merc doesn't, it might move backwards relative to the Ferrari for the same reason, though there isn't a massive reason to believe the Ferrari/Renault will gain more than Merc from engine improvements.
Either way Williams are improving more than most as this year goes on, which is a good sign they understand the car and how to improve it, which bodes well for next year IMHO. The converse of this is Mclaren don't seem to be improving, haven't for two years, the guys in charge seem to have little understand of why the car does/doesn't work and as such don't seem to be improving it an awful lot, which doesn't bode well for next year. Not least because with a different engine, which could be much hotter, or whatever, effectively a lot of packaging things will change and some major aero things may have to change, they are starting from scratch and have been the worst(by a huge margin when you consider budget vs success) team in understanding and getting the best out of a new car. None of that bodes well for getting everything perfect for next year.
The biggest advantage available in the first season is the Merc engine, switching to a non Merc engine car is a big risk.