What the intranet causes, it can also address. Because it allows information to reside all over your company, it can build multiple routes to that information. This prevents one situation from taking the entire company down. The definition of 'reasonable' will have vastly different meanings based on your industry. This is one half of the equation.
Availability is more than just addressing unplanned downtime.
It must focus on planned downtime as well. This includes activities such as operating system updates, backups or any activity which causes the application to be unavailable to the user. Previously, high availability was the mantra of the mainframe or special fault-tolerant solutions. These proved very expensive. Today, organizations can receive the same availability results at a much lower cost with open systems.