The issue with this is that Netflix/Spotify are pay once monthly, and consume whatever you want whenever you want.
With Stadia, you pay once monthly, and then you pay for the game on top of that cost. The same price you'd pay if you bought the game for any other platform, which also has the benefit of letting you own the game (physical copies, files downloaded to hardware you own).
That's just the current payment model, but that is orthogonal to the underlying technology. I can imagine a Spotify Free version of Stadia (yes, with ads or other limits) and a Premium all-you-can-play version. Game demos can be replaced by "Play now" buttons on YouTube, literally dropping you into the game in seconds.
What the platform promises is to match the ease of use of YouTube or Netflix. If it can actually deliver on that, I'm sure we'll see a lot of different business takes on the same technology. As someone who started gaming on an Atari and still maintains a top-of-the-line PC, I see a streaming model as inevitable since games need to compete with the Netflixes of the world for your attention. As a new dad, the barrier (timewise) to actually playing something these days is prohibitive, so Netflix wins by default for me when I have an hour.
With Stadia, you pay once monthly, and then you pay for the game on top of that cost. The same price you'd pay if you bought the game for any other platform, which also has the benefit of letting you own the game (physical copies, files downloaded to hardware you own).
You're comparing apples to oranges.