For those who are interested, a project like this is being kickstarted now:
https://www.kickstarter.com/projects/234135283/antstream-retro-gaming-reborn?ref=bkgayy
More info: https://www.kickstarter.com/projects/234135283/antstream-retro-gaming-reborn/posts/2477226

it's streaming only though, sadly.

By now there already are subscription based gaming offers like EA's Origin Access, Microsoft's Xbox Game Pass, or even streaming subscriptions like Sony's PS Now on PC.

As for one model offering nearly all the games, something like Steam as streaming service, I can't see that happening. The trend goes towards more fragmentation. All the big players will want to have their own streaming/subscription services instead of allowing the competition to make profit with the other's IPs. Which on the other hand will probably prevent any single service from becoming too popular.

You asked for a Spotify-like experience, didn't you? Spotify is a streaming (music) service, just like Netflix etc.

If you wished for something else... then I am unsure what exactly. A Humble Monthly kind of thing where you get more games (licenses) for a set monthly fee?

You can download an album or a playlist with a premium subscription (100 sek per month).