Or, you realize that your processes are so minimalistic and well structured due to lack of options, that you only have to write a custom request router over the weekend to migrate.

Also, Lambda-like options are available with most PaaS providers now.

It is not much different and might be easier than migrating web app from a custom PaaS. The only issue is, and has always been, the migration of data. And I don't see that getting solved until some startup writes a bunch of layers on top of a bunch of providers. It's very tricky, for good reasons.