I think he's been given a chance down the order and still didn't do a lot? I agree though that he should not be a test opener any longer - as you say, his record his shocking. He has 3 Centuries and 16 50s, so it's not even that he's just a poor starter, he doesn't tend to go on to make really big scores if he makes a start. His time may be up (though tbh, many others have been saying that for some while now and he's still there). Which seems weird, as, even with my limited knowledge of NZ cricket (which basically comes from reading people's posts here) there's a few obvious potential replacements around.
Edit: Off Topic somewhat, but just curious. Given the issues that NZ & England are having with their openers, what would be people say would be 'enough' for someone to keep their spot until someone much better comes along? Personally, I could live with someone averaging about 32 as long as they were consistently scoring 25 or so, i.e seeing the shine off the new ball. Of course, I'd probably then get annoyed if they didn't go on, but right now they're not even getting to double figures with much regularity.