Re: Quite honestly, from my perspective ...
The biggest problem with cloud-or-not debates is that it is exceptionally infrequent that apples are compared with apples.
The next biggest problem is that there is no one set of global requirements or priorities such that there can ever be a single answer. We all knew and accepted that when IT was predominantly in-house but it seems that far too many have forgotten that now.
Implementing the same thing in the cloud as in house will almost always always end up costing more in the long run. BUT, it's important to note that in some cases - probably a lot of cases - that's irrelevant. And for good reason (again, in some cases).
One position with cloud solutions that is FAR too common is that once you get rid of the internal kit, it's all now someone else's problem and those someone elses are far better placed to manage it all than a comparatively limited in-house team. The bit about that that can bit one in the rear is that offloading the 'problem' also offloads the control. This is important because all of this magic still runs on software stacks running on hardware platforms with their various firmwares and ancillary components, all of which are interconnected in complex ways.
That software and firmware still needs to be patched and updated and the hardware still needs to be upgraded and replaced and expanded and maintained. It's great that all of that is no longer the concern of the company but it still has to be done and it can - and does - still cause problems.
The key difference is that when you are running a distributed, global cloud, there is no maintenance window and no way to ensure that any operation is schedule so it has the least likelihood of impacting a given workload. Further, with such a large system, any problems that result may take longer to rectify.
Further still, these problems may affect an entire company's global operations whereas internal issues may have only affected one office or region. Take the failures of Adobe's Creative Cloud that had world-wide impacts - what if your entire global publishing house runs on CC? Not quite the same as the type of cloud operators being discussed here but Office365 failures can see whole regions or indeed multiple regions bereft of service. In a decently-implemented multi-region Exchange/AD setup, the loss of one site or even region should need not impact the other sites.
Now, none of this is necessarily a big issue - it's just something that is seldom considered and it is relevant when discussing the various pros and cons and especially when discussing costs because people tend to only see the savings and 'efficiencies' that will come from these migrations and therefore pay less attention to the potential costs.
Cloud services allow many companies to do things they wouldn't otherwise be able to do by enabling them to increase their consumed resources quickly and easily without massive increases in capex. But one must always ask whether the product you are getting really meets your needs.