How come?
IMO there'd be a much higher chance of it causing problems than preventing an attack.
Try to think of an attack that the forum's current HSTS setup wouldn't protect against.
How would it cause a problem? If you don't have the ability to operate using https you simply shouldn't be operating (this goes for any site which needs to secure communication between server and client). If something results in you losing your TLS cert for a period of time it would be better to not operate the site until it is restored. If anything the only useful value for HSTS would be infinite (i.e. NEVER UNTIL THE END OF TIME CONNECT TO THIS DOMAIN INSECURELY) but since that is not an option a very long HSTS value is used as a proxy.
