First of all:
> dedicated on-premise hosting
I did not say on-premise for a reason. Most people are not well placed to host on-premise. For starters it tends to require ops staff on site, which in many countries means a minimum of 3 shifts of a minimum of 2 people. On-premise deployments rarely makes sense.
I said dedicated hosting, which implies renting servers from providers like e.g. Hetzner.
But that said, you can replatform to anything from anything and save money in most organizations, because most organizations tends to be very bad at optimizing cost, so this to me says very little.
Most of the systems I've moved over the years were on the other hand carefully architected to be "cloud friendly" to start with. Some of them started out on cloud platforms and were migrated off to save money.
When you on the other hand start comparing the amount of compute and bandwidth you can get for the same prices, it becomes very clear how overpriced they are.
You can easily find bandwidth at less than 1/10th the price of AWS for example, and in fact I've had clients where their bandwidth bill alone at AWS was bigger than the total hosting bill after I'd moved them elsewhere. No amount of architectural change of their systems will change that - at a minimum you need to reduce the data transfer from their AWS setup. Now, you don't need to move everything out of AWS to fix that - often the savings you can achieve by cutting the AWS bandwidth bill can pay for an entire CDN....
Dedicated hosting also tends to give you far more flexibility in the precise hardware configuration to the point where savings can be similarly huge by substantially reducing the number of instances.
> I believe the business benefit of the agility gains that come from instant and decentralized resource provisioning will always trump any cloud bill...
Nothing prevents you from spinning up cloud instances when needed. Most dedicated hosting providers today also offers cloud instances, so you can typically do that even with a single provider. In practice, the cost difference between dedicated and cloud typically allows a substantial overprovisioning and still saving money, but if you're prepared to use cloud to handle spikes, you can save even more by using dedicated by going closer to the wire, because you know you can spin up cloud instances to take the peaks.
I've set up and operated systems like that which balanced loads over both colo's, dedicated hosting and cloud instances seamlessly several times.