When do you think this will happen (as in having a large library and no lag), and what would you pay?

"Spotify is a Swedish commercial music streaming, podcast, and video service that provides digital rights management–protected[4] content from record labels and media companies.[5] It is available in most of the Americas, Western Europe, and Oceania.[6] Music can be browsed or searched by artist, album, genre, playlist, or record label.

Spotify operates under a freemium business model, with two music streaming tiers: Spotify Free and Spotify Premium. Benefits of the Premium subscription include the removal of advertisements, improved audio quality, and optional downloading of music to devices for offline listening.

Spotify was launched in September 2008 by Swedish startup Spotify AB. As of June 2015 Spotify had more than 75 million active users. The number of paid subscribers reached 30 million in March 2016.[7] Spotify Ltd. operates as the parent company, headquartered in London,[8] while Spotify AB handles research and development in Stockholm.[9]"
https://en.wikipedia.org/wiki/Spotify

Edit:
There's also this, a free, legal library of DOS games: https://archive.org/details/softwarelibrary_msdos_games
In the showcase you can download some of them, not sure if all DOS games are now public domain.

So perhaps the above will be expanded to other platforms in the near future, but some will probably have to be commerical.

That depends; what the hell is it? :P

Not really interested considering I buy here to have games DRM-Free. :P

It was already a thing though. Onlive, it failed.
Online infrastructures can't handle streaming games properly yet for a lot of people.

https://en.wikipedia.org/wiki/OnLive

"Gaming Examiner judged that the graphics were like "playing a PlayStation 3 on a 480p standard [definition] TV", that they thought that they experienced much lower framerates than expected, and that the controller was not working reliably."

Well, after looking up what spotify was - it is a music streaming service - I don't think this is going to happen any time soon. True games publishers would Love to have this in place, to retain total and unrestricted control over the product. But currently it is down to peoples connections. MMO's run by transmitting only a tiny amount of information via the web to the server, the assets are normally on the players machine. Imagine transmitting a 20gb game over the web in real time, its just not feasible. Music is far smaller - mp3 song is a few mb, so can be streamed, youlube buffers the video and that can take a while and mostly they are small files as well.

If you then say, well keep the assets on your machine, a bit like buffering, then that's no different to what we have now from the install. It adds facial features to your characters.

It's streaming?

Okay then, NO! :P

Assuming the experience is the same, and the price is low, why not?

Because the experience won't br the same. Have fun modding a game when you don't have access to any of the files.

Because I get 10Gb of bandwidth a month. :P

a) It isn't the same experience, there will be some latency (especially notable in fighting games) not to say that some hicup on the connection will be noticeable in game;

b) Onlive was exactly that and it failed;

c) PS Now (a recycled Online) shows the type of pricing that you will get with this and i don't think it's being sucessful;

d) With cheap games on promotions and bundles, who will be the audience for this?

e) Spotify isn't really making money.

Valid point, if a bit of a niche concern, but let's say the streaming was an alternative and you can still buy each game if you wanted to, like with Apple Music if I'm not mistaken.

tinyE: Let's say that wasn't a problem. We're talking about the hypothetical future after all.
And with 10+ years old games, would it even be a problem right now?

Streaming games? No thanks.

yeah no
sony has the monopoly on that now and the rates are insane

Modding is a niche concern? Have you tried playing dark souls, or one of the elder scrolls game without mods?

On the second point, as I mention above, if they had a streaming service there is No way they would release any other copy. It boils down to the Gollum Syndrome, same as DRM. Why release a version which might get pirated, keep total control.

Well, if we are talking about a theoretical future I would just be able to send my neural pathways through to Googleplex or M$Ville and interact fully with the virtual world ;o)

For older games, the people who are interested in streamed services aren't generally the people who want to play a 10 year old game.

a) That was one of the OP questions; When you think it will be good enough.

c) I'll have to go check, but that's right now; not in the future.

d)Assuming it would be like Spotify or Apple Music, you would have thousands of games from the entire history of games ready to play just by clicking on them, you could still buy and dl them (and there would be sales as usual), and you would pay either 10$ a month or nothing at all but then have to watch/alt+tab for ads every 30 minutes or so.

Can you explain e)? I found an article about this but it's pretty long.

I rather buy my games. Besides EA is experimenting with a subscription service. I wonder how that goes. Game streaming is on the other hand a no-go. You don't want to wait a half an hour every time you try to launch your game.