Slack has an API, presumably official and non-official.

A large group of hackers likely can figure out a way to export it all...

Rate limits are bad (2/min for channel history). We've explicitly been told not to scrape API, since admins are working on exporting the data into Mattermost.

Don't subscribe to the solutions without data export. And cron the daily export of your data from the solutions you're subscribed to (and better choose the providers with CDC capability). Pure situation of voting with your dollar.

Obvious caveat here - the law of course must be made for monopolies.

A law would be better, otherwise companies will remove data export functionality.

Even a daily export won't save you from the export functionality disappearing with zero notice, because it's really disruptive to try and stop using a service with zero notice. Your company will be left with several weeks if not months of un-exported data.

"This one thing I think is important, and could easily stipulate in a contract, should be law"

People rarely get to actually negotiate contracts with a SaaS company. Unless you’re a very large customer it’s simply not worth their time. Such imbalances regularly give rise to regulations in other parts of the economy see automotive lemon laws etc.

Most SaaS companies can disable data exports at any time. Even if you’re regularly backing up that data when they disable it you need to instantly move to a new service or there’s going to be a gap.