Self hosting makes this situation even worse(see recent VMware stories), as they can quote anything or have different price for the exact piece of hardware or something. SaaS at least has uniformity in price of what they offer and the price for new users and existing users can't be very different.

Again, servers still run while you negotiate.

You seem to forget that there was an agreement about price that slack did not respect.

When this happens in a normal world, you get lawyers and a court involved.