"1. If Frontier wants this game to last for a decade like RCT3, maybe steam is not where they should sell the game. We don't know how much longer steam will be very popular among gamers. I think it will still be popular then but sometimes new and better stuff come along."
The digital game industry is just started and Steam has the most users if you look to other platforms like Origin or Uplay. I think they won't stop in the next 5-15 years.
"2. Having to be online to play Planet Coaster. I think on Steam, you have to be online and connected to wifi to play your games. If I want to play this game on my laptop on a plane ride or bus trip, I have to some how find good wifi that can connect to the steam servers."
In Steam you can play offline games

You can start Steam if you don't have an internet connection. It's up to Frontier to make it playable if you don't have an internet connection. I have hope that Planet Coaster is playable if you don't have an internet connection.
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Using quotes is a bit bugged