Only thing wrong with this decision is the timing - it should have been done a lot earlier. Cook is not and never has been one of the top ten one day English batsman. In my view he has only remained in the side because of the ECB's unfathomable desire not to split the test and one day jobs. Hopefully he can go away and get his Test game back to its previous heights.
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On the plus side should the ECB acquiesce, and I am sure it's their decision and not Cook's, he should be available to help Essex push for promotion in September instead of playing in the ODI series against Australia. Availability through all of June when the New Zealand ODI takes place, is also a possibility. Seven matches in all which would have been more but for the late scheduled West Indies series.
A top four of Cook, Browne, Westley and Ryder is not too shabby especially for Div 2.