Plenty of people would rather take downtime than pay for redundancy, for example for a test database.

AWS RDS lets you spin up a RDS instance that costs 3x less and regularly has downtime (the 'single-az' one), quite similar to this.

Anyone who's used servers before knows "A single instance" is the same as "sometimes you might have downtime".

Computers aren't magic, everyone from heroku (you must have multiple dynos to be high availability) to ec2 (multiple instances across AZs) agree on "a single machine is not redundant". I don't see how fly's messaging is out of line with that. They don't tell you anywhere "Our apps and machines are literally magic and will never fail".