Absolutely they should. But I don't think a rational self-interested company would continue to allow those APIs to be used in order to facilitate a competitor siphoning off users.
I've seen a growing number of people actively moving away from Slack or complaining about it due to these issues. How can Slack fix that? Close off an API to stop hiving off users? Well the explicit intent of this approach is to use both tools so it doesn't seem like they lose much.
Either way; I'd argue that a rational company would look at why it's customers were doing these things and then either a) add that feature to the product or b) let things alone.
Slightly different if it's an "export your Slack chat to A.N.Competitor".