You have to look at a bigger picture for Mercedes. In the financial side, I think this is a calculated move from Bernie to keep Mercedes in F1, with more flexible commercial rights and LH to seal the deal. On the racing side, look what Ross Brawn has finally put together this year for a techincal crew, they arent exactly beginers, and whether they could create a competitive car remains to be seen. But this is what they sold to Lewis and their plans on how to be sucessful. Lewis fears that he will not be able to maintain his current performance level for too long, he is at his peformance peak and he wants to capitalize it so he could obtain that winning "fame".
McLaren mind set to win is not the same as Ferrari or Red Bull or any other teams. It is also their weakness as to why they have not win anything for the past 13 years beside a WDC. Sure they have won more GPs next to Ferrari but over how many decades compare to say RB. I would say McLaren has the best and dare I say even better than Ferrari at times in term of development and engineering which explain why they could recover in such a short time. It is their pit wall operation and strategy which produced some major failures. Like I said above, this is the 3rd time they let a WDC leave the team.