Your comparison makes no sense. Watching a movie in a theater is much like watching a an actual play or a live concert, it’s a “group experience” that isn’t quite like watching a playback on your TV.
If you divide those 870 over 24 months (2 years), that’s 36 dollars a month. If you spent that money every month on games, you could get 1 or more each month and might luck out and get a HUGE “return of investment” if you calculate it entirely as money / time played. Plus, it’d be hours and hours of entertainment any day of the week, not only “each weekend”. Buying on GOG would also mean you get to “own them forever”.
You pledge to support the development of the game
Please act and reply like an investor instead of a fanboy, then. Just because you want to see the game come through doesn’t mean you have to defend everything RSI does to raise money.