What I cannot figure out is the fact that steem being a Blockchain technology is suppose to be immune individual node shutdown.

So why on earth is the whole network going down on a software update?

Surely the update should start with just a couple of nodes and see for a period of time how the software perform before expanding the update to a few more nodes, etc.

A software update must be backward compatible to work with other nodes running older software releases.

Witnesses should coordinate their effort for for a progressive update.