I can make a new website with a forum if Reddit goes "bad," too, but that doesn't change anything
It doesn't change anything on Reddit, because Reddit doesn't federate with anything and nothing can federate with Reddit. People on Reddit would have to create a new account to interact with your forum.
Here on the Threadiverse, if you start a new community on a new instance then the users who were using the community on the old instance can seamlessly move over to start using the new community instead. The content would remain available to both, the users would remain available to both.
Migrating people from one site to another isn't easy
On the Fediverse migrating people isn't necessary, since users on other instances can interact with each other.
This is ultimately the point of Federation. There isn't a "critical mass" for each instance because they all share the same userbase in aggregate. As soon as a new instance comes online they instantly have as many users that can post there as an instance that's been around for years.