It has to be, even with as irrational as the market can be I don't see people sustaining this kind of spend on public clouds -- the cultural barriers and perverse incentives that prevent effective use of private / hybrid cloud have to erode eventually.
That's not borne out by history, especially in the face of "enterprise" hardware, software, and support, which, in many large companies, is being replaced by cloud (at lower cost!).
For smaller companies, it may be a different story, especially at the next economic downturn, especially if VC money becomes scarce enough for long enough.
There are armies of salespeople brainwashing executives about "the cloud". At my last job it only took a few years before every single C-level was spouting off recycled cloud propaganda
I won't go as far as calling it yet another cyclical IT phase, but it has all the hallmarks of one.