Them and the people who see the advertisements for the game at the Six Flags parks:
Even still, not everyone goes to Six Flags. Heck, I haven't been to a Six Flags park since 2001. They will reach more people with a new trailer on youtube, as well as some TV ads.
I'll check out those livestream videos was well.
And yes, Planet Coaster looks like a polished RCT3, but at least Frontier is getting it right the first time. I think Atari should've hired them and made Planet Coaster the new RCT for PC, but I'm not disappointed with what NVizzio has showed off so far. I just need to see more... Lots more.