You are right that the technology borrows a lot from the past. In fact, you may call this a sort of "pattern" for shared system computing. However, I think enough different elements of the experience have changed significantly to call this "new":

- The business model is subscription or demand based. (Then again, perhaps the old timeshare models pioneered this...)

- The marketing model is focused on the consumer as much as the enterprise. (That is definitely new...)

- There is no fixed limit to the scale of systems running in the cloud, though most vendors limit long running or intensely large scale computations. (Perhaps the big difference here is you aren't getting a fixed time block, but rather a "cushion" in which you can grab a tremendous amount of compute capacity as needed then give it back.)

- A new architectural model in which computing crosses both institutional and political boundaries without the knowledge of the end user. You hint at this, but this is really where most of the challenges seem to be coming from these days.

Another way to look at this is we may finally be seeing the dream of an online market for commodity compute services come to pass. That definitely was not true for the mainframe era.