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".
Just because some customers are less fault tolerant than others, doesn't mean we shouldn't offer those options where people don't have the same requirements or are willing to work around it.
If this sounds ludicrous, then I think I probably don't understand who Fly.io wants to be and that's okay. If I don't understand, however, you may want to take a look at your image and messaging to potentially recalibrate what kind of customers you're attracting.