I don't equate codify and automate. That's sometimes a piece of it (automating state changes), but more specifically I'm thinking about all of the validations, human processes, etc that go into customer and sales management. I'd need to think about how slack can "codify communications", but I'm sure there's some people that already have answers for this.

SFDC has one of the most robust APIs I've seen...you can export pretty much everything. I actually really enjoy working with the soql rest API, because it's simple, effective, and discoverable.

The lock-in is instead in their integrations. It is almost guaranteed that there won't be another platform that has all of the integrations you need, so you stick with SFDC. You add more integrations over time and the lock-in deepens.