But if Vettel doesn't win the title, then Alonso's decision is still vindicated - Ferrari still haven't won a title since 2008 or a drivers for a decade.
Alonso was 6th in the championship in 2014 (and Raikkonen 12th!). While McLaren were further back, the potential of Honda coming in with all the design and engineering changes made the move a risk, but he was coming from a team in turmoil and struggling with no obvious signs of a big improvement, to a team that in theory had massive potential for the future. Clearly it hasn't worked out, but up until this year at least he still wouldn't have won a title or even come close.
This is not really what happened anyway, but even if it was it was obviously flawed. First up lets mention that was it 44 point lead in 2012. Had Alonso not squeezed Kimi in Japan Alonso wins the title in 2012, that car was fully capable, he had a great car. The RBR finished the season great but so what, the Brawn was dire in the second half of the season and still won the title. Alonso had a huge lead and his own personal mistake cost him a great position and a great shot at the title. People like to conveniently forget that the Ferrari was a 'meh' qualifier but had excellent race pace in several of those years.
Past that, Ferrari had made a title competitive car in recent history(2012), Ferrari were putting a lot of money and time into the engine, though they got the design wrong. Throughout 2014, before he left, Ferrari brought in huge engine people from Merc, from Lotus(Who had the guy who did the previous like 7 years of ERS R&D for Merc) and lots of other people, they were on the up and were better than Mclaren. Mclaren made a stupid deal that at the time looked stupid to bring a rushed engine from a team not involved in F1 for years and not successful in F1 for much longer than that. Nothing about it screamed good chance, everything about it screamed utter disaster incoming.
As for the seemingly fairly real story of what happened. Alonso wanted out of Ferrari believing he could get a RBR or Merc seat, he started negotiating hardball with Ferrari, the rumour being he asked for a role in team management, ie involvement in high level team decisions. The theory is he asked for this precisely because he thought they'd tell him to **** off and let him out of his contract. What he wasn't betting on was Ferrari, RBR and Merc all knowing he was available and all making a move of their own to cut him out. He had another year contract at Ferrari still but two drivers the team were going to put ahead of him. He had one big team left and went to Mclaren.
I'm no fan of Alonso but in any way does Mclaren's history of dire tactical choices, huge mistakes that kept costing Hamilton, his own personal history with Ron, Honda's recent history in multiple motorsports let alone F1 all being bad and knowing that Honda would be rushing the engine or Mclaren making two terrible cars in two years, sound like a good idea or a real chance to succeed in the short term future? Anyone who thinks Alonso actively chose Mclaren and thought they'd do well has a very low opinion of Alonso. He's not that stupid, every single sign was pointing at failure, the only reason Alonso would go there would be no other options.
What he says to excuse that decision is pretty much irrelevant. He's not going to say hey, I screwed myself out of continuing at Ferrari, Merc and RBR shut me out and despite the fact that it looks like a bad idea I guess I'll go to Mclaren for the money? Politics, he plays off going to Mclaren as a personal choice and comes up with a story about why it's a great idea.