In the end, this decision works out best for all three drivers and the team.
When you have three drivers vying for two seats, it's inevitable that someone is always going to get their feelings hurt. Despite the ridiculous amount of delays and the agonizing tug-of-war that has plagued this highly anticipated announcement, in the end, McLaren-Honda made the right call. And I say that for everyone involved, including Kevin Magnussen.
All the right pieces in the right places
It's no surprise that the team will put most of its weight on Fernando Alonso but It's also good to see that McLaren had enough respect to not simply throw a still very capable Button to the kerb like a used rag. He'll have one last shot at glory here and will be placed on the same stage as arguably the best driver in F1 today. And with Honda back in play, McLaren has the prospect of potentially becoming the formidable opponent they once were, but I don't expect to see instant success. They have all the right pieces in the right places for them to claw their way back to the top though.
Strongest driver pairing in F1
McLaren has what I believe is the strongest driver pairing in the field now. Besides the fact both are very successful World Champions, their personalities compliment each other as teammates. They both know how to get the most out of whatever they have, usually pushing it beyond its limits and making it work. They also understand how to finish races, which is crucial in any form of motorsport.. They have the instinct that allows them to know exactly when to hold back and when to take risks. You won't see any clash of the egos here either, something McLaren has come much too familiar with facing.
The Mercedes' pair are liable to turn on each other without warning, Ferrari has two drivers that I feel can't take a car anywhere beyond its capabilities, and Red Bull has a young talent, who is now the 'veteran' of the team as an even younger talent joins him in 2015. I see no weakness in the 2015 McLaren driver pairing.
On the McLaren side of it, they need veterans such as these two wheeling their machines in order to properly analyze their program as they embark into uncharted territory with Honda. Also, there won't be any shenanigans you'd have to worry about if you paired a young driver with Alonso. Naturally, he would want to prove himself against such a titan and solidify his place in the sport. The last thing McLaren needs right now is a team dynamic such as that.
They have the drivers and the technical partner, but will they succeed?
As for Magnussen, he may have been demoted, but if he is patient enough (and McLaren decides he is a better fit than Vandoorne), he will get his seat back once Button hangs up the helmet.
In the end, this deal is the best outcome for McLaren, Honda, and all the drivers involved. They have what they need to succeed, now the pressure is on McLaren to perform, because people won't be blaming the drivers when they don't...