Make free play weekends available for pro and non pro, and advertise this heavily to make people at the least try stadia.
There are Play Free Days already on occasion. See here.
You do need to be a Stadia Pro subscriber to take advantage of them, though. I think they don't want to open Stadia up for any truly free, zero commitment, play yet as there is a large potential for abuse there. Anybody can try Stadia right now by signing up for a free Stadia Pro trial.