Will be cool to look at, but I don't think we're through with the mainframe era just yet. They are starting to become more power conscientious that their blade counter parts, from what I understand.
Kindof agree here, there's actually some good use cases for mainframes and in certain niche applications I hear people advocating for them again (after a long period of these-dinosaurs-are-on-their-way-out).
Also think that in some sense the lines are blurring between mainframes and huge clusters of commodity servers. Yes, you can build extremely large/powerful clusters of commodity servers now (glorified pc's basically) and have a google-style compute farm, but not many people have the resources to do that and maintain it. You are now seeing large established companies (including IBM and the like, ironically) who will happily build you a big cluster, supply all the networking and high-I/O interconnects, set up the clustering software, and support the whole thing. At that point, you're essentially back to a mainframe model that spans several racks that is probably lacking in the ability to scale vertically.
A lot of assumption here on my part, of course, as I'm only going by what I know of mainframes, but I think there still seems to be a place for vertically-scaled systems.