Re:Redundancy...
People have forgotten how mainframes work. The vast majority of today's PC users are most probably ignorant of what mainframes even are.
A PC is motley collection of basic hardware components. Its advantage is that it is easy to make components for it, which is why the hardware industry is wallowing in innovation. PCs are used for single tasks most of the time, and the user can generally not be bothered by knowing what is actually going on in the thing.
A mainframe is a delicate, precise assemblage of components tailored specifically to suit much more demanding hardware specifications. Mainframes can warn you in advance if just about any one of their components is going to fail - so you can replace it before it dies. Mainframes generally allow you to unload and switch off the area you do maintenance in, so that you do not need to shut down the mainframe to replace a component (ever heard of hot-swappable ?). With RAID, PCs have come to learn what hot-swappable is in the disc arena (but not users). Mainframes can do that with just about anything but the backplane (a motherboard, in other words).
In other words, I'm sorry to destroy your illusions but when the 3900 PCs are turned off, the few mainframes will easily take up the position and you're going to wait a long, loooong time before getting a day off because one of the mainframes went down. If it is competently managed, you might never see it be down during your entire career.