This should put things in perspective in regards to the viable nature of cloud gaming. It also speaks volumes about the success of competing services. Not to mention that how cloud gaming is implemented really matters to gamers.
One reason for NVIDIA's success might be that it offers both a free tier and a paid tier. But it would naive to think that how it serves up games over the cloud has no impact.
GeForce NOW subscribers get access to their game library
No doubt that one of the biggest parts of GeForce NOW's popularity is the game access. It doesn't just offer users a large game library. Though it does do that.
It offers access to nearly any game they own. Chances are if you bought it, you can play it through GeForce NOW. Save for games from some key publishers. This is not to be understated. Because while services like Stadia do (or at least will) offer up access to new games, so will GeForce NOW.
In fact NVIDIA confirmed that it will allow access to Cyberpunk 2077 the same day as it launches on other platforms. Which means it's more or less on par with everyone else in terms of games you can play.
It's no wonder why so many people were willing to sign up for the service this quickly. Because it gives them a way to play games on the go that are already on their PC. There are a lot of benefits to this.
Destiny 2 is a good example. If you play Destiny 2 on PC then you're playing through Steam. And since GeForce NOW lets you play your Steam games, you can boot up Destiny 2 and play not only with your entire clan, but the entirety of the PC player base.
Some of those subscribers upgraded to the paid Founders tier
To be clear, its subscriber numbers aren't all free tier users. NVIDIA says a number of them upgraded to the paid Founders tier within the last week.
That means those players started with free and then decided to pay the $5 monthly fee to gain access to the Founders perks. Which includes RTXON graphics and priority access. In addition to six-hour long sessions instead of one.
It's also likely that the playtime limit is a big reason why some users decided to upgrade. One hour sessions are fairly short especially if you play games often and usually for longer stints.
If you haven't given GeForce NOW a try yet, you should.