I'm a contractor on about the 4th cloud migration project for a worldwide organization. I'm not an expert on the cloud, but here's my two cents.
As usual, the migration I'm part of is just a 'lift and shift' operation (meaning they want the cloud to run just like their data center). They are following the IaaS model.
On one hand, it makes sense for a world-wide organization to use the cloud for the availability and redundancy. On the other hand, just lift and shift is the easiest going to be the most expensive, (easy=expensive).. The real cost savings, if they are to be had, would be to refactor the databases, webservers, and so on to take advantage of the cloud infrastructure, depending on whether you want IaaS, PaaS, or SaaS. Only with SaaS could you get away with a minimal IT staff and IaaS would need much more staff. Maybe using the cloud native databases or serverless would work better and cost less overall than IaaS and your IT staff still running 'classic' databases and webservers. Or what about containers?
Lift and Shift might be a good way to start to keep your company running, but then there needs to be the effort (and budget) to refactor your code and data to take advantage of the cloud based on how you want to run it.
Like I said, not an expert but learning. Every time I read a story like this, I am left with a feeling of 'they' haven't really explored how to best use the cloud. The project I am currently on is the third one to go to AWS. In studying up on AWS they have many tools to track your usage and billing, and many tools to estimate your costs. And there are caveats on which storage you use and how you want to move your data around and what that will cost.
I would like to see some case studies by people who are familiar with what it takes to operate an on-premises set-up versus and properly configured cloud setup. Such a case study would hopefully give metrics on what it costs and how much effort to refactor existing code into a cloud project that maximizes efficiency/cost savings (for a true comparison, rather everyone jumping on the latest hotness and then jumping off six months later after their half-assed effort didn't work out)