If Mitchell really has done most of the work with the horse then I find the decision understandable.
The sad thing to me is that knowing as they did that Mitchell had already made plans to set up on his own this year - he was meant to go straight after Cheltenham but waited for the Alners' sakes - they didn't offer to give him the licence, given the severity of Robert Alner's injuries, and let him run the yard himself initially with their help and advice. Then ALL the horses would have stayed, and the situation could have been the best solution all round for everyone.
I doubt very much that Mitchell was setting out to pinch horses while Alner lay on his deathbed - most owners know very well what goes on in a yard and often talk to the HL or Assistant about their horse for another viewpoint, so I doubt they needed to be 'seduced'. If Mitchell has had the principal care of the horse for all the time he's been in the yard, they would know who is responsible for his success
I wouldn't be surprised if quite a few Head Lads aren't better 'horsemen' than their trainers, but lack the social skills in many cases, not to mention the capital and the self-confidence, to set up on their own. A wise trainer makes sure his key staff has full reward and recognition. In any case Mitchell was Assistant, always a step to setting up in training, and made his plans clear long before the accident