Depends on how deeply the firms have gotten locked into specific cloud provider services. It's possible to do things in a cloud-agnostic way (for example, Terraform-- ugh), but those implementations are suboptimal, and just as they used to say about Java's much-touted OS independence, it's "write once, debug everywhere."

I'm convinced that it's better to optimize for the cloud service you're running on, and take the one-time hit if you move somewhere else. And I don't believe there's much rearchitecting involved: it'll be lower-level stuff, mainly IaC and other config. If you have to recode, you probably did it wrong the first time.

From my software development standpoint that could actually be. AWS and azure have a hefty user lock in that could cost a lot to migrate out of

Yeah, going from AWS to Hetzner will require doubling your SRE/platform/infra/whatever-you-call-them staff, clearly.