On-prem or cloud, why would I be waiting until 1 month before launch to provision anything?
You're pretending like on-prem is full of all these physical limitations, but Amazon, Microsoft, Google, Oracle, et. al. can just magically make hardware appear out of nowhere. It's the same hardware, subject to the same supply limitations, and I'm going to be paying either way. Either I pay more per hour for my cloud instances, or I pay a bunch up front to provision my on-prem servers.
Cloud makes sense if the load on your service is intermittent or unpredictable enough that you're okay paying AWS's premiums in order to have the ability to scale on demand. But if the load on the services is knowable, then hosting that service on the cloud is paying a 25% markup for no reason whatsoever. AWS is by far the most profitable portion of Amazon's business units. Those profits are coming from somewhere. They're coming from your business.
You do raise a fair point - what about the applications that are more economical to host on-prem? In my own portfolio I estimate 20% or so of my applications make more sense to host on-prem. What to do about those?
The most practical option is to mandate every application you build in the cloud have a plan for how to run on-prem. Note those that would be very difficult to run on-prem - those are your key cloud dependencies. For example, in my own portfolio I have a couple of applications depending on Lex and therefore would be very difficult to host on-prem.
Pay special attention to your so-called "crown jewels" - your apps that distinguish you from your competitors. If those were dependent on a key technology such as Lex and would be difficult to migrate on-prem then you need to identify other cloud providers you could migrate to. In this particular example I know Microsoft's and Google's clouds both offer similar services.
If you design and build your applications with no thought of the future then you're already in a bad position.