Supposedly Button only signed yesterday, supposedly Alonso signed months ago, supposedly Magnussen already had a multi-year contract. Ron's comments and what we think we know appear to me that Button have been the problem here, a preference by team and Honda, not willing to sign on proposed contract, being in a position where he could force their hand enough to get a contract better than what they intended to offer, and being strong enough backed by either board or Honda to not fear being dumped.
I expect Alonso to come out on top.
Why Alonso is leaving a team, willing to build everything around him for an under performing team and a new engine does not really mesh with the stated object of Alonso to win more WDC's than what he already has. Unless his view of Ferrari is that they are so much in the doldrums, that a McLaren Honda partnership will better equip him to battle for the elusive 3rd, 4th, 5th Championship he so crave.
I am a firm believer in not having parity between drivers, there should be a clear 1 and a clear 2, McLaren have set themselves up for driver discontent in same way Mercedes managed last year, Kimi and Sebastian at Ferrari be turn out to be a team of flawed Champions, which considering them holding 5 between them is a strange thing to think.
With the ongoing saga as it have been, the other driver moves have to be part of the whole discussion here, Vettel to Ferrari was mooted last year, that may have started Alonso moving towards somewhere else, action and reaction we will be in for some interesting interteam battles, with the McLaren one being a very interesting to follow.