Slack has raised our charges by $195k per year

That's a 40x increase all at once with a very short grace period, it's bait-and-switch territory.

If only 2.5% of targets pay the ransom, Slack breaks even on this racket, so in absence of any protection this strategy is most likely profitable for Slack.

This is something you pull if you want to squeeze in the short term, and don't mind losing customers.

Was not obvious before but these days it is: choosing VC-backed service is very risky.