Nobody claims that Gengar+Flying type will give free switches forever and ever, but they will make you guess wrong every once in a while (say 50% of the time), creating opportunities. Rhydon won't always Earthquake due to the existence of Exeggutor, so staying in with your paralyzed Chansey/Snorlax/anything becomes a better option. It won't always prevent you from tanking that Earthquake with said Chansey/Snorlax/etc. but it will make you guess wrong and as such, make your situation more favorable than it'd otherwise be. Same thing with Gengar+Flying type.
And as such, you stay in and soak Body Slam for 0% damage (potentially waking up), since nobody EQ's a sleeping Gengar. It works both ways.