And many many users will leave, as they did with Digg. It's too large and lowest common denominator already, and a major redesign will be the impetus for a brain drain as all users will have a shared gripe. HN is a very likely spillover target, and already is for a lot of the programmer focused discussion that used to be Reddits core user base. I've watched so many communities die from monetization, the cycle really writes itself.
Eventually even HN will get too bloated and need to fracture.
I definitely think there is a risk. However, reddit is much larger now than Digg was then. And reddit existed as a clear alternative to Digg at the time of the redesign.
I think there's a very real risk of a redesign going wrong, but it may be large enough and better enough than alternatives to weather the storm.
"Better enough" is a funny thing when it comes to redesigns, because all the previous routes to value that are ingrained into your brain are now gone, or look funny/different. Thats why I predict a brain drain as people pick up and realize it really doesn't do much for them anymore.
Eventually even HN will get too bloated and need to fracture.