I’ve shared my feelings at length on the lackluster premium features several times in other places, so I’ll just comment on this from the “ensure Beeminder continues existing” angle.
As a rule I am usually happy to contribute nominally to services I like. I use Patreon.com, I used to pay HabitRPG $5 a month, etc. But Beeminder is a different beast with the pledge system.
I would be willing to pay $5 a month if that money accrued into an account which offset future derails. That account could provide up to, say, 50% of the derail funds so if I owe a $30 derail I could draw $15 from my monthly fee account. In this way, I pay a bit less for derails but 1) they can never be free and 2) the next-level pledge will still be a scary deterrent.
Using such a system, I know that if I never derail I’ll pay the $5 per month but if I derail enough my total spending will approximately equal the total derails since the monthly fee is entirely consumed in the limit.