Agree with the sentiment here. On top of this, something very important are integrations.

We use a lot of tools that send messages to dedicated Slack channels for notifications. CI failures, incidents, etcs. They use probably Slack API that you can replicate, but the integrations are native in other services ("Click to connect to Slack"). Without that, you are in a big disadvantage.

But good luck!

replicating the api is a great idea!