A stable company where growth as a percentage isn't likely to be significant can run things cheaper on their own in most cases. At least if you consider the cost of the inevitable departure from the cloud provider either to switch another or to go on-prem. And if you aren't willing to make that exit, you can guarantee your cloud provider won't stop cranking up the fees until the threat of you leaving surfaces.
You basically run the base load in your own data center and the surges go to the cloud. My university is evaluating this because sometimes you have multiple labs that need a lot of compute resources at the same time and local compute cluster has finite capacity.
But hopefully you don't get trapped in the cloud and can claw the margin back.
Ok, let's make one thing patently clear: ITS THE SAME IN THE CLOUD
All the cloud vendors will tell you need to have stuff replicated in multiple "Availability Zones" or "Regions".
And yup, the nickle & diming nature of the cloud means that's going to cost you double or triple.
Bandwidth is the one thing where the cloud clearly wins wrt idle servers... except that DC bandwidth is a hundred times cheaper than AWS bandwidth, so you should prefer buying 133% or 150% or 200% DC bandwidth by a mile.
You can redirect traffic to another region and have autoscaling spin up EC2 instances, etc.
But, very notably, you can have a suspended cloud backup even if your main servers aren't cloud. And the added complexity for datacenter-to-cloud HA doesn't have to be significantly higher than the cloud-to-cloud version.
If you complicated your architecture enough - and the cloud makes it oh so easy to make rube goldberg architecture - keeping many different services running or even developing in-house can take a lot.
And it's not like cloud costs you zero in ops work either, just need different set of skills.
But it is not like on-prem stagnated - there is plenty automation in that space too. Our team of 3 manages 7 racks of servers and few dozen projects on them (anything from "very legacy" to 30+ node k8s cluster and ceph storage with it) and the hardware management still isn't majority of our work