The main reason we haven't offered a perpetually free account is because we're a bit different than other SaaS companies: hosting isn't our only cost, we also have to pay for each phone and SMS alert we send.
The other reason is that we see PagerDuty as solving a real "hair on fire" problem, and we think if you're one of the businesses that needs this, it's reasonable to pay a certain amount for the service. I'd like to hear your thoughts on this.