I've been thinking about it before. I've got thoughts on what would happen after 5 years or so, but not 20-30 years.
In 5 years, I think PC will still be here, and I'll still be here (lurking), but in 20-30 years, there will probably be an event to cause PC to shut down, as I saw Yamikarasu post.
In terms of Pokemon losing popularity, ending or anything of the sort, I don't think that would completely stop PC from continuing. People who like to go here because of the community part will still hang out and be around. But It'd greatly remove a whole lot of new users to come, I'm sure of that.
It's possible that PC will still be around after that long, as long as the people don't get tired of being here.