I think I understand your concern but if I miss the point, please follow up!
So regarding getting access to read knowledge from the different tools, it depends tool by tool but a lot of them have API keys or options for app integrations available in the free tier (GitHub, Google Drive, Confluence come to mind). Other tools don't have a free tier and you just get access to the API keys as a part of paying for the service. I think there are probably tools that require a premium fee to get integration access but I'm not aware of any personally.
For the SlackBot, it can add itself to public channels but for private channels someone needs to add it. It is what it is sadly.
About search being available for most SaaS products: SaaS tools are definitely improving their own searches. But I still think a single place to search and aggregate data has significant value. For example, as an engineer by training, often getting the full picture for some customer escalation includes reading Slack threads, Confluence Design docs, old Pull Requests on GitHub. Would be nice to get it all in one place.