If you'd keep Magnussen over Alonso its probably a good job you're not in a position to make that kind of decision
It's easy to say that. You're not taking into account the possibility of Magnussen developing and Alonso declining. We saw it happening rather quickly with Grosjean and Raikkonen. Alonso in 2014 wasn't as good as Alonso in 2012. The first signs of one's speed dropping is his qualifying performances. That dropped dramatically for Alonso in recent seasons. He has changed his style into one based more on consistency and intelligence than on explosive pace. That aspect is gone. Going back to Raikkönen, last season he was leading the championship out of consistency in the first couple of races. Grosjean was nowhere to be seen. Then Grosjean found a way to get his explosive pace from 2012 back without crashing all the time and Raikkönen couldn't keep up at all. And Kimi's driving style, how it developed over time, is looking scarily similar than Alonso's, except for it happening faster. So assuming that the first real shot at winning the title happens in 2016, by that time Magnussen could've been in a position to regularly beat Alonso. That is, if McLaren truly believed in his reputation as a future World Champion. Apparently they don't. Sensible long-term planning would've meant sticking by Magnussen at all costs.
Also, if 2015 is going to be a year of development, Button would be more suitable than Alonso, who in his years at Ferrari never got the repuation as a driver that contributes greatly to the development of his car as the season progresses. Alonso is purely the driver they need to get results, but in 2015, there is a big chance they won't be in a position to get winning results on a regular basis anyway. They don't desperately need Alonso, but his reputation means it's irresistible to pass up on him. By demoting Magnussen, McLaren is acknowledging that they messed up and that trying to do a Red Bull is something they cannot do. It makes Hamilton's success look more like a fluke for their Academy. And signing Alonso is yet again a risk that could backfire real hard on them. Their relationship has been rocky in the past to say the least and as Vettelfan showed, it's not that farfetched to think he won't jump ship for 2016 because at this point he's desperate to be in the best car.
So long story short, 2015 is probably going to be more about development, so keeping Button for that, while also keeping Magnussen to lead them in the seasons beyond 2015 would be the best choice. Now we have a situation where you don't know if Alonso is willing to stay for much longer than a year and whether Magnussen is ready by the team he does leave.
If anything Alonso is getting better all the time. He's a better driver than he was when he won his championships. You say Alonso isn't known for being good at developing a car, but is Button? What has JB done to develop the McLaren over the last 2 years, if anything the McLaren was worse this year than 2013. Also, of course Alonso wants a Mercedes. Saying he could jump ship at the end of the year if a Mercedes seat appears is kind of silly. If Kevin or Jenson were offered a McLaren or a Mercedes at the end of 2015 they'd both take the Mercedes as well
Alonso in terms of outright speed isn't the as good as 2006 I wouldn't have thought. In terms of intelligence yeah probably 2014 Alonso is better. He probably peaked between 2010-2012 though IMO. I don't really see how someone over the age of 32 can be "improving all the time". Sure, he is still the best on the grid but Hamilton will surpass him on that one very soon IMO.