I see. Have you considered eliminating this configuration from your offering? It sounds like the terminology could confuse people, and it may be the case that they're assuming that a host isn't really what it is (a single host). This kind of thing is difficult for those seeking to build managed services, because I think people expect you to provide offerings that can't harm them when the cause is related to the service they're paying for and it's difficult to figure out which sharp objects they understand and which ones they don't. People should know better, but if they did would they need you?
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.
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".
Sure, but isn't this more about risk tolerance at this point and how much your customers care about? Where the responsibility should be on customer's end. Running on EBS/RDS doesn't guarantee you won't lose data. If you care about it, you enable backups and test recovery.
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.