Martin's isuse with Game of Thrones is he's been working on it for 26 years without breaking away to do anything to refresh himself. He wants to get it done and keep the fans happy. He certainly wants it to be done more than any reader does.
Has he really not broken away at all (genuine question) ? I've been visiting his Not-a-blog every now and then for a few years and there's always a new Wildcards book freshly edited. He may not write them himself, but that still seems like a ton of work. Recently he even announced another 5-years contract with HBO for additional GoT universe series. The passion is still burning and the man is always working on something. Just, not A Song of Ice and Fire (which I can understand after TV told the entire story already). At this point I'm tempted to think that, on some level, he himself has given up on it.
Back to the topic, I'm not sure if I would want to see someone else pick up Berserk. Publish whatever chapters were already done or in progress at the time of Myura's passing, sure. But unless he left precise notes on where he would have headed the story, I'd prefer for it to remain ufinished.