The issue isn't about an OS being much more than a forum (whatever that means), the issue is actually size of potential market. There are millions of users who would use an OS, but do you think the size of market is same for forums as well? Software doesn't cost high because it costs high to make it, actually it costs higher because the company making it has to survive and make profit (given the total size of market). (Or, of course, it can start making OSes).