In reality, the cloud creeps into your systems in all sorts of ways. Your permissions use cloud identities, your firewalls are based on security group referencing, your cross-region connectivity relies on cloud networking products, you're managing secrets and secret rotation using cloud secrets management, your observability is based on cloud metrics, your partners have whitelisted static ip ranges that belong to the cloud provider, your database upgrades are automated by the cloud provider, your VM images are built specifically for your cloud provider, your auditing is based on the cloud provider's logs, half the items in your security compliance audit reference things that are solved by your cloud provider, your applications are running on a container scheduler managed by your cloud provider, your serverless systems are strongly coupled distributed monoliths dependent on events on cloud specific event buses, your disaster recovery plans depend on your cloud provider's backup or region failover capabilities, etc. Not to mention that when you have several hundred systems, you're not going to be moving them all at the same time. They still need to be able to communicate during the transition period (extra fun when your service-to-service authentication is dependent on your cloud) without any downtime.

It's not just a matter of dropping a server binary onto a VM from a different provider. If I think about how long it would take my org to move fully off of _a_ cloud (just to a different cloud with somewhat similar capabilities), 3 years doesn't sound unrealistic.

IMO, the value proposition these days is rather to avoid maintenance. I.e. help with up with all the latest patches on your infrastructure.