If you don’t have a high degree of consensus for protocol upgrades the network can totally break down.
Imagine the network protocol updates at 51%. Depending on which shareholders update you could lose half or more of the total minting power. Now you have two separate networks without the same minting power prior to the hard fork. Guess what? The difficulty does not take that into account. It could become far more difficult to mint blocks (if any get minted at all). There will be huge numbers of orphans broadcast to each node as they figure out who to ban which can have its own ill effects. This is a really broad explanation but there are a lot of technical reasons why you need a high degree of consensus when performing a protocol upgrade. You are forking the network and there are many factors to consider related to the security of the network as a whole.
Even the core developers of the “Bitcoin Classic” proposed hard fork suggested a 90% “hash rate voting” for the fork to become valid. You can see references to is here by searching “90%” on the page.